docs: adding transparent background configuration

[germa89]

Description

Using transparent background in images to avoid contrast with page background.

Issue linked

3172

Checklist

• [x] 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.
• [x] I have reviewed my changes before submitting this pull request.
• [x] I have linked the issue or issues that are solved to the PR if any.
• [x] I have assigned this PR to myself.
• [x] I have marked this PR as draft if it is not ready to be reviewed yet.
• [x] 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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[germa89]

I need to check the online docs.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 80.21%. Comparing base (cda716a) to head (a87150c). Report is 2 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #3233 +/- ## ========================================== - Coverage 82.64% 80.21% -2.44% ========================================== Files 53 53 Lines 9625 9623 -2 ========================================== - Hits 7955 7719 -236 - Misses 1670 1904 +234 ```