Add Particle Emitter tutorial

[iche033]

🎉 New feature

Summary

Adds a tutorial on how to spawn and configure particle emitters in ign-gazebo

Test it

To build the doc locally, go to the build directory and run make doc. See the tutorial by opening main index page doxygen/html/index.html, go to Tutorials, then click on the Particle emitter page.

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

[codecov]

Codecov Report

Merging #860 (bd28098) into ign-gazebo4 (31676f5) will not change coverage.
The diff coverage is n/a.

@@             Coverage Diff              @@
##           ign-gazebo4     #860   +/-   ##
============================================
  Coverage        65.69%   65.69%           
============================================
  Files              240      240           
  Lines            17797    17797           
============================================
  Hits             11691    11691           
  Misses            6106     6106           

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