docs: add Context Hub webhook guide

[Will Bradley (wbbradley)]

Why

Context Hub commit webhooks now have a workspace-wide settings experience and a stable delivery contract. This guide makes the shipped configuration, signing, retry, deduplication, and v1 payload behavior available without requiring readers to inspect the implementation.

What changed

• Add the Context Hub commit webhook guide and navigation entry.
• Link the guide from the Context Hub overview and UI workflow.
• Document workspace scope, Fleet-originated commits, management behavior, delivery semantics, and every v1 payload field.
• Extract the existing Engine webhook signature examples into one reusable snippet shared by both webhook guides.

Review carefully

• Confirm the Settings navigation and management behavior match the landed UI from #30422.
• Confirm retry classification, payload fields, and optional-field behavior match the backend implementation.
• Confirm extracting the Engine signature examples into a shared snippet preserves their rendered presentation.

Validation

• Python and TypeScript signature harnesses: valid signature accepted; tampered body, malformed header, and incorrect signature rejected.
• make lint_prose FILES="src/langsmith/context-hub-webhooks.mdx src/langsmith/context-hub.mdx src/langsmith/use-the-context-hub.mdx src/langsmith/engine-webhooks.mdx src/snippets/langsmith/webhook-signature-verification.mdx"
• uv run --group test codespell on the touched MDX files
• make build
• mise exec -- make broken-links-with-anchors

AI involvement

This PR was implemented with assistance from an AI coding agent. The agent reviewed the landed backend and UI source, authored the documentation changes, ran the code-example harnesses and documentation checks, and prepared this PR description.

[github-actions]

Thanks for opening a docs PR, Will Bradley (@wbbradley)! When it's ready for review, please add the relevant reviewers:

• @katmayb or @fjmorris (LangSmith)

[github-actions]

Mintlify preview branch generated: preview-wbbrad-1784329971-1348faf

Site preview: https://langchain-5e9cc07a-preview-wbbrad-1784329971-1348faf.mintlify.site

Important

Preview links may take a few minutes to start working while the deployment finishes.

Changed documentation pages (preview deep links):

• src/langsmith/context-hub-webhooks.mdx
• src/langsmith/engine-webhooks.mdx
• src/langsmith/context-hub.mdx
• src/langsmith/use-the-context-hub.mdx