Switch to PyData Sphinx theme and add a landing page

After merging various parts of Software Heritage documentation in a single Sphinx instance, the navigation using the Read The Docs theme became seriously cumbersome.

After reviewing several Sphinx theme, we switch to PyData Sphinx theme: a clean, Bootstrap-based Sphinx theme by and for the PyData community. For details, please refer to: https://pydata-sphinx-theme.readthedocs.io/

We also implement the landing page that was designed during the documentation sprint in November 2022.

There is probably further work to be done on the copy, maybe tweak colors and margins on the cards. Proper support for dark mode would be nice as well. It’s a bit ugly for now.

• Advances: swh/meta#4844 (closed)
• Supersedes: !267 (closed)
• Obsoletes: !313 (closed)