Document release process

[danehans]

Description

Adds a maintainer release guide for this repo and links to it from the root README.

Docs changes

• Added RELEASE.md with repo-specific release instructions
• Documented the relationship between tags, GitHub releases, Cloud Build, and release-triggered workflows
• Added a README link to the release guide

Context

The release flow was not documented in one place, which made it harder to understand how to cut a release.

Interesting decisions

The guide focuses on the process encoded in the repo today rather than a desired future workflow.

Testing steps

• Reviewed the existing release-related configs and workflows referenced by the guide
• Ran git diff --check
• No automated tests were run because this is a docs-only change

Notes for reviewers

Please sanity-check the operational steps against the current release tooling and access patterns.

Checklist:

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works

[github-actions]

Visit the preview URL for this PR (updated for commit bedc6f6):

https://gloo-edge--pr11199-release-docs-lcxq1zhy.web.app

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