docs: renovate README to reference doco site

The current README is a mish-mash, and needs to be split up into a proper guide.

Author: jamestelfer

README.md

@@ -1,239 +1,43 @@
-# Chinmina Bridge: Buildkite/Github OIDC token bridge
+# Chinmina Bridge
 
-[![FOSSA Status](https://app.fossa.com/api/projects/git%2Bgithub.com%2Fjamestelfer%2Fchinmina-bridge.svg?type=shield)](https://app.fossa.com/projects/git%2Bgithub.com%2Fjamestelfer%2Fchinmina-bridge?ref=badge_shield)
+**Connect Buildkite to GitHub with secure, short-lived tokens.**
 
+Chinmina Bridge allows Buildkite agents to securely generate GitHub API tokens
+that can be used to perform Git or other GitHub API actions. It is intended to
+be an alternative to the use of SSH deploy keys or long-lived Personal Access
+Tokens.
 
-Allows Buildkite agents to get valid GitHub tokens that can be used to perform
-Git or other GitHub API actions. It is intended to be an alternative to the use
-of SSH deploy keys or long-lived Personal Access Tokens.
+![High level Chinmina diagram](docs/chinmina-high-level.png)
 
 The bridge itself is an HTTP endpoint that uses a [GitHub
 application][github-app] to create [ephemeral GitHub access
 tokens][github-app-tokens]. Requests are authorized with a [Buildkite
 OIDC][buildkite-oidc] token, allowing a token to be created just for the
 repository associated with an executing pipeline.
 
-The token is created with `contents:read` permissions, and only has access to
-the repository associated with the executing pipeline.
-
-Two endpoints are exposed: `/token`, which returns a token and its expiry, and
-`/git-credentials`, which returns the token and repository metadata in the [Git
-Credentials format][git-credential-helper].
+> [!NOTE]
+> Find out more about Chinmina Bridge is available in the [documentation][docs].
+>
+> This has and expanded [introduction][docs-intro], a [getting
+> started][docs-started] guide and a detailed [configuration
+> reference][docs-config]. This has a more detailed description of the
+> implementation, and clear guidance on how to configuration and installation.
 
 [github-app]: https://docs.github.com/en/apps
 [github-app-tokens]: https://docs.github.com/en/apps/creating-github-apps/authenticating-with-a-github-app/generating-an-installation-access-token-for-a-github-app
 [buildkite-oidc]: https://buildkite.com/docs/agent/v3/cli-oidc
 [git-credential-helper]: https://git-scm.com/docs/gitcredentials#_custom_helpers
 
-## Overview
-
-`chinmina-bridge` is used by jobs running on a Buildkite agent to request tokens
-from Github. These can be used to communicate with the GitHub API or (via Git)
-to enable authenticated Git actions.
-
-Git authentication is facilitated by a [Git credential
-helper](https://github.com/jamestelfer/github-app-auth-buildkite-plugin), which
-communicates with the bridge and supplies the result to Git in the appropriate
-format.
-
-The following sequence illustrates a Git authentication flow facilitated by
-`chinmina-bridge`.
-
-```mermaid
-sequenceDiagram
-    box Buildkite Agent
-        participant Buildkite Job
-        participant Git
-        participant Credential Helper
-    end
-    box Self hosted
-        participant Chinmina Bridge
-    end
-    Buildkite Job->>+Git: clone
-    Git ->>+ Credential Helper: get credentials
-    Credential Helper->>+Buildkite API: Request Buildkite OIDC token
-    Buildkite API->>-Credential Helper: bk-oidc
-    Credential Helper->>+Chinmina Bridge: Request GH token (auth bk-oidc)
-    Chinmina Bridge->>+Buildkite API: Get Pipeline Details
-    Buildkite API-->>-Chinmina Bridge: pipeline-repository
-    Chinmina Bridge->>+GitHub: Create Token (auth app JWT)
-    GitHub-->>-Chinmina Bridge: app-token
-    Chinmina Bridge->>-Credential Helper: bk-oidc
-    Credential Helper->>-Git: "x-access-token"/app-token
-    Git-->>-Buildkite Job: complete
-```
-
-## Why?
-
-There are two options generally used to authenticate Buildkite agents to GitHub:
-
-1. Via a PAT (owned by a GitHub user) that is saved in the agent S3 secrets bucket
-2. Via a deploy key (registered to a single repository) that is likewise saved to
-   S3.
-
-As the organization scales however, the overhead of managing them becomes
-unwieldy, and it can be quite difficult for an organisation to successfully
-manage a rotation scheme.
-
-Unless centralized issuance is practiced as well, both of these schemes can
-produce tokens that are tied to a user, leading to unexpected problems when a
-user leaves the organization. There is also the potential for key material to be
-stored or shared incorrectly, leading to increased possibility of accidental
-leakage.
-
-Lastly, all key material is typically stored in an S3 bucket. This is
-straightforward to configure and maintain, but creates a significant issue in
-the event of an account/bucket breach.
-
-Using a GitHub application to authenticate GitHub actions allows:
-
-1. Access keys for repositories are generated on demand and expire after one
-   hour.
-1. The generated tokens are only kept by a build agent for the duration of the
-   step, and do not require any other persistence.
-1. The private key for the GitHub application is specific to the
-   `chinmina-bridge` service. It can (and should) be rotated, an operation that
-   is easy to perform.
-1. Supplied tokens are scoped to just the repositories and actions necessary for
-   the requesting pipeline.
-1. Additional Buildkite configuration per repository is not required. If the
-   application has access, the agent can request a token for it. No need to
-   create PATs or generate keypairs, and no need to upload them in multiple
-   places. This allows the an organization to have tighter access control on
-   pipeline setup without creating additional support overhead.
-1. Tokens can enable a wider set of actions than simple Git operations (e.g. PR
-   comments). This is not yet implemented in `chinmina-bridge`, but is a high
-   priority for future enhancement.
-
-Also, since `chinmina-bridge` uses Buildkite's OIDC tokens to authorize requests,
-the claims associated with the token can be used to further refine access to a token.
-
-Github has some [good documentation][gh-deploy-keys] about the pros and cons of
-the application token approach. There are two primary downsides documented:
-
-> - Additional setup is needed to create the GitHub App.
-> - Installation access tokens expire after 1 hour, and so need to be
->   re-generated, typically on-demand using code.
-
-`chinmina-bridge` solves the second problem, by making token generation for a
-pipeline at build time trivial.
-
-
-[gh-deploy-keys]: https://docs.github.com/en/authentication/connecting-to-github-with-ssh/managing-deploy-keys#github-app-installation-access-tokens
-
-### What's right for your organization?
-
-To understand what's right for your organization, consider:
-
-- how many pipelines do you have? (That is, how many keys are managed?)
-- how easily are tokens rotated?
-- (related) if the secrets bucket is somehow compromised, how difficult would it be for the organization to respond?
-- if tokens are issued to a user, does a person leaving cause an outage in a build pipeline?
-- what processes/restrictions does your organization have around repository access in GitHub and pipeline creation in Buildkite?
-
-## Limitations
-
-- can only grant `contents:read` access
-- will only grant access to the repository associated with a pipeline
-- if the buildkite user has permissions to modify the pipeline repository, they
-  may configure a repository that they don't have access to in GitHub (but is
-  accessible in the app). This would allow them to potentially extract code via
-  use of the pipeline step configuration. **BUT**:
-  - it's OK if your organization members have read access to the same set of
-    repositories covered by the `chinmina-bridge` GitHub application.<br>
-    **OR**
-  - it's OK if your organization controls the creation/configuration of
-    pipelines: this restricts the opportunity to misconfigure a pipeline.
-
-## Operations
-
-See the [observability documentation](./docs/observability.md) for more details
-on the information provided by the system when running.
-
-## Configuration
-
-Requirements:
-
-1. A Buildkite organization, and a user with sufficient access to create an API
-   token that can be used to get the details of any pipeline that is expected to
-   be built.
-1. A Github organization, and a user with sufficient permissions to create a
-   Github App and install it into the organization.
-1. Ability to deploy a server that can be accessed by the build agents (for example, an ECS service)
-1. Ability to allow Buildkite agents to download and use a custom plugin _or_
-   ability to add a plugin to the default settings of the Buildkite agents.
-
-### Buildkite setup
-
-Create an API key with access to the REST API **only** with access to the `read_pipelines` scope.
-
-Save the key securely: it will be provided to the server in a later step. Use a
-"bot" user to create the token if you can.
-
-### Github setup
-
-1. Create an application in your Github organization
-    - The application must request `contents:read`
-    - Note the application ID
-    - Create and save a private key for the application
-2. Install the application into the Github organization
-    - choose the repositories the application will have access to. This is the
-      limit of the resources that the application can vend tokens for.
-
-### Configure and deploy the bridge server
-
-The server is a Go application expecting to read configuration from environment
-variables, and can be deployed to a server or as a container.
-
-#### Variables
-
-**Server**
-
-- `SERVER_PORT` (optional, default `8080`): the TCP port the server will listen on.
-- `SERVER_SHUTDOWN_TIMEOUT_SECS` (optional, default `25`): the number of seconds
-  the server will wait when asked to terminate with `SIGINT`
-
-**Authorization**
-
-- `JWT_BUILDKITE_ORGANIZATION_SLUG` (**required**): the slug of your Buildkite
-  organization. This is the identifier of your organization that appears in your
-  Buildkite URLs.
-- `JWT_AUDIENCE` (optional, default=`app-token-issuer`): The expected value of the
-  `aud` claim in the JWT. Describes the intended audience of the issued JWT
-  token, guards against token reuse. Using a non-default value will require configuration of the credentials helper plugin.
-- `JWT_ISSUER_URL` (optional, default `https://agent.buildkite.com`): the
-  expected value of the `iss` claim in the agent JWT. Also used to discover the
-  JWKS configuration from the `.well-known` address.
-- `JWT_JWKS_STATIC` (optional): a local JWKS JSON file that can be used instead
-  of Buildkite. Used to verify the JWT sent by the Buildkite agents to the
-  server. This should only be required for server testing, as agents will only
-  create a token using the Buildkite key.
-
-**Buildkite API**
-
-- `BUILDKITE_API_TOKEN` (**required**): The API token created for pipeline
-  metadata lookups. **Store securely and provide to the container securely.**
-
-**GitHub API connectivity**
-
-- `GITHUB_APP_PRIVATE_KEY` (**required**): The PEM formatted private key of the
-  created Github app. **Store securely and provide to the container securely.**
-  This is a highly sensitive credential.
-- `GITHUB_APP_ID` (**required**): The application ID of the Github application
-  created above.
-- `GITHUB_APP_INSTALLATION_ID` (**required**): The installation ID of the
-  created Github application into your organization.
+[docs]: https://chinmina.github.io
+[docs-intro]: https://chinmina.github.io/introduction/
+[docs-started]: https://chinmina.github.io/guides/getting-started/
+[docs-config]: https://chinmina.github.io/reference/configuration/
 
 ## Contributing
 
-Contributions are welcome.
-
-- `direnv` is the tool for setting up the test environment
-- some variant of docker compose makes it easier to run locally
-- Run `make keygen` to create test keys
-- Execute `git` commands in the `.development/keys` directory. This has git
-  configuration set up so it uses a local credential helper that will use the
-  keys in the `.development/keys` directory.
+This project welcomes contributions! Take a look at the outstanding issues for
+something to dip your toes into, open an issue to get some input, or raise a PR
+if you're confident.
 
 ## License