[Backport maintenance/3.3.x] Ref #10260 -- Add documentation for the github output format

[github-actions]

Backport 5f26f32 from #10272.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.84%. Comparing base (6b68f91) to head (a034f0b).
Report is 23 commits behind head on maintenance/3.3.x.

Additional details and impacted files

@@                Coverage Diff                 @@
##           maintenance/3.3.x   #10273   +/-   ##
==================================================
  Coverage              95.84%   95.84%           
==================================================
  Files                    174      174           
  Lines                  18982    18982           
==================================================
  Hits                   18193    18193           
  Misses                   789      789           

Files with missing lines	Coverage Δ
pylint/lint/base_options.py	100.00% <ø> (ø)

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