Add CODEOWNERS defaults and governance guidance

[BreakableHoodie]

Summary

• keep the documentation refresh from PR docs/community doc refresh #2 in this branch so every handbook file matches what is already on main
• add a default reviewer list (GitHub's CODEOWNERS file) so organizers automatically review any repository that does not have its own maintainer list yet
• describe which project teams look after each repository today, who still needs a team, and the risks of leaving repos without active reviewers
• lay the groundwork for org-wide branch protection rules by explaining, in plain steps, how reviews and required checks will work

Testing

• not applicable (documentation-only)

Supersedes

• closes docs/community doc refresh #2

[chatgpt-codex-connector]

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[BreakableHoodie]

Thanks for the flag! GitHub does let us use a CODEOWNERS file in the .github repository as the fallback for repos without their own file. The docs call this out in the "Code owners and organization repositories" section: https://docs.github.com/en/repositories/managing-your-repositorys-settings-and-features/customizing-your-repository/about-code-owners#codeowners-and-organization-repositories. We verified that behaviour before drafting this guidance, so the default file here will apply org-wide unless a repo overrides it locally.