chore: let docs approvers approve mkdocs.yml

[blakepettersson]

@argocd-approvers-docs cannot approve #15689 since mkdocs.yml is not included in the CODEOWNERS file. This should be pretty benign to add.

Checklist:

• Either (a) I've created an enhancement proposal and discussed it with the community, (b) this is a bug fix, or (c) this does not need to be in the release notes.
• The title of the PR states what changed and the related issues number (used for the release note).
• The title of the PR conforms to the Toolchain Guide
• I've included "Closes [ISSUE #]" or "Fixes [ISSUE #]" in the description to automatically close the associated issue.
• I've updated both the CLI and UI to expose my feature, or I plan to submit a second PR with them.
• Does this PR require documentation updates?
• I've updated documentation as required by this PR.
• Optional. My organization is added to USERS.md.
• I have signed off all my commits as required by DCO
• I have written unit and/or e2e tests for my change. PRs without these are unlikely to be merged.
• My build is green (troubleshooting builds).
• My new feature complies with the feature status guidelines.
• I have added a brief description of why this PR is necessary and/or what this PR solves.

[blakepettersson]

cc @34fathombelow

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (f37d24f) 49.54% compared to head (7e4e750) 49.56%.
Report is 8 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #16134      +/-   ##
==========================================
+ Coverage   49.54%   49.56%   +0.01%     
==========================================
  Files         269      269              
  Lines       46641    46668      +27     
==========================================
+ Hits        23109    23131      +22     
- Misses      21259    21263       +4     
- Partials     2273     2274       +1     

see 6 files with indirect coverage changes

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.