docs: add release automation docs

Author: polyipseity

docs/release.md

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+# Release automation (summary)
+
+Releases are produced by `googleapis/release-please-action` in `.github/workflows/release.yml`.
+
+- Trigger: push to `main`.
+- Behavior: `googleapis/release-please-action` parses Conventional Commits and will open/update Release PRs or create a release when appropriate; it also generates/updates the changelog (`CHANGELOG.md`) and exposes outputs such as `release_created`, `major`, `minor`, `patch`, `tag_name`, and `body`.
+- CI integration: the workflow uses those outputs to tag versions and to build/push container images (see the repo's workflow for the exact steps).
+
+## Notes for maintainers
+
+- Review the draft release or Release PR produced by the action, add concise highlights or migration notes, then publish/merge.
+- Do not commit a handwritten `CHANGELOG.md` to the repository; rely on per-release notes generated by the action.