Revise CODEOWNERS to use entire TW team

In !801 (merged), @vyaklushin had to ask me to review a change to a Markdown file. I'm happy to do it, and this repository rarely needs TW reviews, but let's avoid creating a single point of failure. Let's set the CODEOWNER for Markdown files to be the entire TW team, rather than just @brendan777.

I stole the idea from the CODEOWNERS file in the charts repo: https://gitlab.com/gitlab-org/charts/gitlab/-/blob/master/.gitlab/CODEOWNERS?ref_type=heads#L39 but I wonder if we should also add these lines:

## Files that are excluded from required approval
## These rules override the * rule above, so that changes to docs and templates
## can be merged by any user who has maintainer rights, but is not included in
## the * rule (usually technical writers).
/.gitlab/issue_templates/*.md
/.gitlab/merge_request_templates/*.md
/doc/*.md

Please double-check my usage. I can't guarantee I got this right.

Related

• Related to Rename bin/check to bin/gitlab-shell-check to t... (!801 - merged) where we found the problem