Improved doxygen

[ahcorde]

🦟 Bug fix

Fixes #

Summary

Improved doxygen, This PR gazebosim/gz-cmake#183 in ign-cmake2 will fix more warnings.

I didn't activate the linter because there are still some warnings to fix.

Checklist

• Signed all commits for DCO
• Added tests
• 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

[codecov]

Codecov Report

Merging #996 (05cde58) into ign-gazebo3 (d94d585) will not change coverage.
The diff coverage is 50.00%.

❗ Current head 05cde58 differs from pull request most recent head 78e009b. Consider uploading reports for the commit 78e009b to get more accurate results

@@             Coverage Diff              @@
##           ign-gazebo3     #996   +/-   ##
============================================
  Coverage        77.84%   77.84%           
============================================
  Files              221      221           
  Lines            12687    12687           
============================================
  Hits              9876     9876           
  Misses            2811     2811           

Impacted Files	Coverage Δ
include/ignition/gazebo/EntityComponentManager.hh	100.00% <ø> (ø)
include/ignition/gazebo/Server.hh	100.00% <ø> (ø)
include/ignition/gazebo/ServerConfig.hh	100.00% <ø> (ø)
include/ignition/gazebo/Util.hh	100.00% <ø> (ø)
...nclude/ignition/gazebo/components/ContactSensor.hh	100.00% <ø> (ø)
include/ignition/gazebo/components/Factory.hh	96.05% <ø> (ø)
...nclude/ignition/gazebo/components/LogicalCamera.hh	100.00% <ø> (ø)
...nclude/ignition/gazebo/components/Serialization.hh	90.32% <ø> (ø)
include/ignition/gazebo/gui/GuiSystem.hh	0.00% <0.00%> (ø)
src/gui/plugins/modules/EntityContextMenu.hh	100.00% <ø> (ø)
... and 10 more

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[chapulina]

This is great! Just some smaller comments.