DOC: Improve documentation

[prisae]

There are many things that could be improved:

• Add version switcher: https://pydata-sphinx-theme.readthedocs.io/en/latest/user_guide/version-dropdown.html
• Adjust images for dark theme
• Build docs on GHA with a docs-branch
• In the gallery, the panels seem to be broken (click does not do anything): Fixed by DOC: move from sphinx-panel to sphinx-design #173