Add tutorial on migrating the Sensor class from gazebo classic

[iche033]

🎉 New feature

Part of #325

Summary

Add migration tutorial for Gazebo classic's Sensor 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 #1930 (1c8708a) into ign-gazebo6 (ef71b65) will decrease coverage by 0.01%.
The diff coverage is n/a.

❗ Current head 1c8708a differs from pull request most recent head 56700c6. Consider uploading reports for the commit 56700c6 to get more accurate results

@@               Coverage Diff               @@
##           ign-gazebo6    #1930      +/-   ##
===============================================
- Coverage        65.31%   65.31%   -0.01%     
===============================================
  Files              327      327              
  Lines            26893    26878      -15     
===============================================
- Hits             17566    17556      -10     
+ Misses            9327     9322       -5     

see 5 files with indirect coverage changes