chore: auto-request docs manager review via codeowners

[ClumsyAdmin]

Summary

Every PR in this repo should be reviewed by JustAMatt, the docs manager, but nobody is requested as a reviewer automatically right now. That depends on whoever opens the PR remembering to add them.

This adds a CODEOWNERS file that makes JustAMatt the owner of every path. GitHub then requests their review on every PR targeting master once it is ready for review. This uses GitHub's built-in feature instead of a workflow: it needs no Actions run or token, and it also works on PRs from forks, where a workflow token is read-only. JustAMatt already has write access, which CODEOWNERS requires.

Changes

• Add .github/CODEOWNERS with * @JustAMatt

Test plan

• After merge, open a PR and confirm JustAMatt is requested as a reviewer automatically
• Optional: turn on "Require review from Code Owners" in master branch protection if their approval should be mandatory (this PR doesn't change it)

No user-visible docs change, so there are no screenshots.

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