Add/commit access docs

[jeffpaul]

What?

This PR adds a CODEOWNERS file and docs on how to get added as a Codeowner and then as a Maintainer.

Copilot summary

This pull request updates project documentation and repository ownership to clarify the code review and merging process, and to ensure proper assignment of code owners for different parts of the repository. The most important changes are:

Repository Ownership and Code Review Process:

• .github/CODEOWNERS: Added a global fallback code owner (@WordPress/ai-maintainers), specified ownership for documentation and project management directories, and clarified who is responsible for code reviews.

Documentation Updates:

• docs/DEVELOPER_GUIDE.md: Added detailed sections describing the PR merging process, steps for joining the ai-maintainers team, and guidelines for closing PRs, including communication best practices and a template for closing PRs.

Why?

How?

Use of AI Tools

AI assistance: none besides Copilot in the PR description

Testing Instructions

Screenshots or screencast

Before	After

Changelog Entry

Developer - Added docs on becoming a Maintainer.

[github-actions]

The following accounts have interacted with this PR and/or linked issues. I will continue to update these lists as activity occurs. You can also manually ask me to refresh this list by adding the props-bot label.

If you're merging code through a pull request on GitHub, copy and paste the following into the bottom of the merge commit message.

Co-authored-by: jeffpaul <jeffpaul@git.wordpress.org>
Co-authored-by: dkotter <dkotter@git.wordpress.org>

To understand the WordPress project's expectations around crediting contributors, please review the Contributor Attribution page in the Core Handbook.

[jeffpaul]

@dkotter I tried to write up some notes on getting folks elevated to Maintainer after reading through how Gutenberg handles things and trying to get some specifics into our docs to help quantify/qualify the process. Curious about your input here and whether I've over- or under-described things?

[github-actions]

✅ WordPress Plugin Check Report

✅ Status: Passed

📊 Report

All checks passed! No errors or warnings found.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 77.05%. Comparing base (8303620) to head (b1917fa).

Additional details and impacted files

@@            Coverage Diff             @@
##             develop     #857   +/-   ##
==========================================
  Coverage      77.05%   77.05%           
  Complexity      2233     2233           
==========================================
  Files            103      103           
  Lines           9216     9216           
==========================================
  Hits            7101     7101           
  Misses          2115     2115           

Flag	Coverage Δ
unit	77.05% <ø> (ø)

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[dkotter]

@dkotter I tried to write up some notes on getting folks elevated to Maintainer after reading through how Gutenberg handles things and trying to get some specifics into our docs to help quantify/qualify the process. Curious about your input here and whether I've over- or under-described things?

This looks good to me. Can adjust as needed but this is a good place to start