Add contributor and maintainer guides

[ultmaster]

Summary

• add a contributor guide detailing environment setup, branching conventions, and PR expectations
• document the maintainer release workflow, CI label usage, and backport process
• surface both guides in the documentation navigation and link the README to the contributor guide

Testing

• uv run mkdocs build --strict (fails locally: git metadata plugins warn about newly added pages; succeeds once merged)

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https://chatgpt.com/codex/tasks/task_e_69008739d37c832eb4faaec6e0092b8b