DOCS: Add thumbnails manually

[SMoraisAnsys]

@lorenzovecchietti This is a WIP to add thumbnails manually:

• pros: only requires one comment in the example and it does not appear when the documentation is built
• cons: we require this comment in the example (might not be something that we want to keep with respect to users)

Another approach could be to add an "expected" result at the beginning of the example. That way, since the expected result is plotted at first, it will be used as thumbnail.

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[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 80.74%. Comparing base (dadf67c) to head (4abe7cd).
Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4647      +/-   ##
==========================================
- Coverage   80.84%   80.74%   -0.11%     
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  Files         119      118       -1     
  Lines       54529    54469      -60     
==========================================
- Hits        44083    43979     -104     
- Misses      10446    10490      +44     

[SMoraisAnsys]

Closing this as changes are going to be performed in pyaedt-examples with another approach