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
• [x] In the gallery, the panels seem to be broken (click does not do anything) [Fixed in emg3d-gallery]
• [x] Replace deprecated sphinx-panels by sphinx-design [#307]