First pass at updating CODEOWNERS

[mhucka]

This reorganizes the file slightly to

• try to make it easier to find things (e.g., by not splitting up docs and code in completely separate sections)

• put the rule for docs/** higher, because the way it was before, it would have overwritten the more specific settings

• use rooted paths, to avoid accidentally matching subdirectories (e.g., because there's a docs in dev_tools/ too)

I also removed a couple of users who either have left Google or have not been involved for a long time.

[github-actions[bot]]

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