Cleaning up documentation

[padix-key]

This issue tracks small tasks, that would improve the look-and-feel of the documentation:

• [x] Update sphinx-gallery version
• [ ] Fix missing Fork me on GitHub image
• [ ] Update Twitter logo to X logo
• [x] Add reference to recent Biotite paper

Common functionalities could be also put in a separate repository and be loaded as submodule. This way also Biotite extension packages would benefit from it.