Add Doxygen documentation checks to CI

[chapulina]

🎉 New feature

Summary

Requires gazebo-tooling/action-gz-ci#30

Also fixed the existing warnings.

Test it

The checks can be run locally by:

cd <build folder>
make doc
bash <(curl -s https://raw.githubusercontent.com/ignitionrobotics/ign-cmake/ign-cmake2/tools/doc_check.sh)

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

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[codecov]

Codecov Report

Merging #225 (fc57a24) into ign-math6 (2c4fa24) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           ign-math6     #225   +/-   ##
==========================================
  Coverage      99.21%   99.21%           
==========================================
  Files             65       65           
  Lines           6089     6089           
==========================================
  Hits            6041     6041           
  Misses            48       48           

Impacted Files	Coverage Δ
include/ignition/math/Cylinder.hh	100.00% <ø> (ø)
include/ignition/math/Ellipsoid.hh	100.00% <ø> (ø)

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