Upgrade to sphinx 4

[Gustry]

Sphinx has upgraded to a new major version Sphinx 4
https://github.com/sphinx-doc/sphinx/releases

We should keep in mind to update the infrastructure.

[Gustry]

According to gitlab in the logs when building the documentation, we are running v1.8.5.

The HTML is different between using Sphinx 1.8.5 and 4.X, for instance bullet lists. So when we upgrade, we would need to check the output and fix some RST formatting.

CC db3e0e0