Populate plugins that were empty

[chapulina]

🦟 Bug fix

• Part of Cleanup GUI plugin menu gz-sim#1254

Summary

Populate plugins that were previously empty so that users aren't too confused when inserting them from the menu.

CameraTracking

Besides displaying message:

• Add top-level plugin docs

InteractiveViewControl

Besides displaying message:

• Add plugin title

MarkerManager

Besides displaying message:

• Make sure stats_topic works even when there's no _pluginElem
• Update docs

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

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[codecov]

Codecov Report

Merging #366 (678065e) into ign-gui6 (70255cc) will increase coverage by 0.04%.
The diff coverage is 63.33%.

@@             Coverage Diff              @@
##           ign-gui6     #366      +/-   ##
============================================
+ Coverage     65.38%   65.43%   +0.04%     
============================================
  Files            39       39              
  Lines          5117     5123       +6     
============================================
+ Hits           3346     3352       +6     
  Misses         1771     1771              

Impacted Files	Coverage Δ
src/plugins/marker_manager/MarkerManager.cc	60.51% <60.71%> (+0.51%)	⬆️
...interactive_view_control/InteractiveViewControl.cc	17.88% <100.00%> (+1.35%)	⬆️

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