docs: adapt static images to dark/light themes

[germa89]

Description

Change background in some static images. I am keeping the pyvista generated background.

Issue linked

Close #3172

Checklist

• I have visited and read Develop PyMAPDL.
• I have tested my changes locally
• I have added necessary documentation or updated existing documentation.
• I have followed the PyMAPDL coding style guidelines.
• I have added appropriate unit tests that also ensure the minimal coverage criteria.
• I have reviewed my changes before submitting this pull request.
• I have linked the issue or issues that are solved to the PR if any.
• I have assigned this PR to myself.
• I have marked this PR as draft if it is not ready to be reviewed yet.
• I have made sure that the title of my PR follows Commit naming conventions (e.g. feat: adding new MAPDL command)

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 84.17%. Comparing base (d4fe3a1) to head (3205960).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3249      +/-   ##
==========================================
- Coverage   86.61%   84.17%   -2.45%     
==========================================
  Files          53       53              
  Lines        9630     9630              
==========================================
- Hits         8341     8106     -235     
- Misses       1289     1524     +235     

[clatapie]

LGTM!