Revise CODEOWNERS for documentation ownership

[bmorelli25]

Summary

Updated CODEOWNERS to better reflect ownership of this repository. Open to suggestions from all.

We could update the writer ownership parts to the elastic/docs team but previous feedback was that it was too noisy for some folks.

Inspired by #3286 which should be owned by docs eng and not writers.

What changed:

• Docs Eng will own the codebase.
• Writers and Docs Eng will share ownership of config files. This allows writers to move without approval from docs eng in release situations.
• Writers must approve readme changes (my least passionate change in this PR).

cc @elastic/docs-repo-owners and @elastic/docs-engineering.

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[colleenmcginnis]

Looks good. One non-blocking question: should writers also co-own extra/? Not sure if any of these files are even still relevant other than extra/docs_landing.html with the current redirect situation.