[envprovider] Add README file (#12499)

<!--Ex. Fixing a bug - Describe the bug and how this fixes the issue.
Ex. Adding a feature - Explain what this achieves.-->
#### Description

Use mdatagen for the env provider. This switches the package test to be
generated and adds an autogenerated section to the readme.

I've also populated the readme with basic usage information.

<!-- Issue number if applicable -->
#### Link to tracking issue
Fixes
#12465

confmap/provider/envprovider/README.md

@@ -0,0 +1,38 @@
+# Environment Variable Provider
+
+<!-- status autogenerated section -->
+| Status        |           |
+| ------------- |-----------|
+| Stability     | [stable]  |
+| Distributions | [core], [contrib], [k8s], [otlp] |
+| Issues        | [![Open issues](https://img.shields.io/github/issues-search/open-telemetry/opentelemetry-collector?query=is%3Aissue%20is%3Aopen%20label%3Aprovider%2Fenvprovider%20&label=open&color=orange&logo=opentelemetry)](https://github.com/open-telemetry/opentelemetry-collector/issues?q=is%3Aopen+is%3Aissue+label%3Aprovider%2Fenvprovider) [![Closed issues](https://img.shields.io/github/issues-search/open-telemetry/opentelemetry-collector?query=is%3Aissue%20is%3Aclosed%20label%3Aprovider%2Fenvprovider%20&label=closed&color=blue&logo=opentelemetry)](https://github.com/open-telemetry/opentelemetry-collector/issues?q=is%3Aclosed+is%3Aissue+label%3Aprovider%2Fenvprovider) |
+
+[stable]: https://github.com/open-telemetry/opentelemetry-collector/blob/main/docs/component-stability.md#stable
+[core]: https://github.com/open-telemetry/opentelemetry-collector-releases/tree/main/distributions/otelcol
+[contrib]: https://github.com/open-telemetry/opentelemetry-collector-releases/tree/main/distributions/otelcol-contrib
+[k8s]: https://github.com/open-telemetry/opentelemetry-collector-releases/tree/main/distributions/otelcol-k8s
+[otlp]: https://github.com/open-telemetry/opentelemetry-collector-releases/tree/main/distributions/otelcol-otlp
+<!-- end autogenerated section -->
+
+## Usage
+
+The scheme for this provider is `env`. Usage looks like the following:
+
+```text
+env:NAME_OF_ENVIRONMENT_VARIABLE
+```
+
+To use default values when the environment variable has not been set, you can
+include a suffix to specify it:
+
+```text
+env:NAME_OF_ENVIRONMENT_VARIABLE:-default_value
+```
+
+Environment variables must match the following regular expression. That is, they
+must be at least one character, start with a letter or underscore, and can only
+include letters, numbers, and underscores.
+
+```text
+^[a-zA-Z_][a-zA-Z0-9_]*$
+```

confmap/provider/envprovider/metadata.yaml

@@ -0,0 +1,8 @@
+type: env
+github_project: open-telemetry/opentelemetry-collector
+
+status:
+  class: provider
+  stability:
+    stable: [provider]
+  distributions: [core, contrib, k8s, otlp]

confmap/provider/envprovider/provider.go

@@ -1,6 +1,8 @@
 // Copyright The OpenTelemetry Authors
 // SPDX-License-Identifier: Apache-2.0
 
+//go:generate mdatagen metadata.yaml
+
 package envprovider // import "go.opentelemetry.io/collector/confmap/provider/envprovider"
 
 import (