Focal CI: static checkers and doxygen linters

[chapulina]

🎉 New feature

Summary

Add cppcheck, cpplint and doxygen checks.

Test it

Focal CI on GitHub actions should return green.

Checklist

• Signed all commits for DCO
• Added tests
• Added example and/or tutorial
• Updated documentation (as needed)
• Updated migration guide (as needed)
• codecheck passed (See contributing)
• All tests passed (See test coverage)
• While waiting for a review on your PR, please help review another open pull request to support the maintainers

Note to maintainers: Remember to use Squash-Merge and edit the commit message to match the pull request summary while retaining Signed-off-by messages.

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[codecov]

Codecov Report

Merging #230 (cb30517) into ign-msgs5 (8595461) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           ign-msgs5     #230   +/-   ##
==========================================
  Coverage      85.27%   85.27%           
==========================================
  Files              9        9           
  Lines            903      903           
==========================================
  Hits             770      770           
  Misses           133      133           

Impacted Files	Coverage Δ
include/ignition/msgs/PointCloudPackedUtils.hh	84.21% <ø> (ø)

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[osrf-triage]

This pull request has been mentioned on Gazebo Community. There might be relevant details there:

https://community.gazebosim.org/t/new-ignition-releases-2022-03-25-fortress-edifice-citadel/1343/1