Convert Ert to use mkdocs rather than sphinx

[pinkwah]

Ert uses sphinx, which is a very broadly used documentation software. However, there are pain points, like slow build times, no live reload and use of ReStructured Text (subjective).

Instead, I would like to use mkdocs, which has the mkdocstring plugin that lets one easily document most everything in Python docstrings, with live reload. A notable user is Pydantic.

One difficulty is implementing documentation from Ert plugins, where it is provided at runtime and in RST format rather than mkdocs' Markdown.

[dafeda]

I don't have strong opinions on this but sphinx-autobuild can be used to get live reload as explained here:

https://github.com/equinor/ert/blob/main/CONTRIBUTING.md#build-documentation