Add tutorial on migrating the Actor class from gazebo classic

[iche033]

🎉 New feature

Part of #325

Summary

Add migration tutorial for Gazebo classic's Actor class.

Checklist

• Signed all commits for DCO
• Added tests
• Added example and/or tutorial
• Updated documentation (as needed)
• Updated migration guide (as needed)
• Consider updating Python bindings (if the library has them)
• 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.

[codecov]

Codecov Report

Merging #1929 (254f47a) into ign-gazebo6 (ef71b65) will decrease coverage by 0.02%.
The diff coverage is n/a.

❗ Current head 254f47a differs from pull request most recent head 16f468c. Consider uploading reports for the commit 16f468c to get more accurate results

@@               Coverage Diff               @@
##           ign-gazebo6    #1929      +/-   ##
===============================================
- Coverage        65.31%   65.29%   -0.02%     
===============================================
  Files              327      327              
  Lines            26893    26878      -15     
===============================================
- Hits             17566    17551      -15     
  Misses            9327     9327              

see 7 files with indirect coverage changes