[ci] Trigger docs checks on merge_group too

[ricab]

If there are incompatibilities between separate changes to docs, we want to detect that before merging.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.27%. Comparing base (bd106dc) to head (ca6a692).

Additional details and impacted files

@@                      Coverage Diff                       @@
##           skip-docs-checks-on-unrelated    #3995   +/-   ##
==============================================================
  Coverage                          89.27%   89.27%           
==============================================================
  Files                                259      259           
  Lines                              14653    14653           
==============================================================
  Hits                               13081    13081           
  Misses                              1572     1572           

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[ricab]

Meh

[ricab]

FWIW actions/runner#1182

[xmkg]

Meh

YAML without anchors? What a shame. GitLab supports it, and it's a pretty handy thing.