Docs/mkdoc

[PGijsbers]

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Work converting the Python docs to mkdocs. Left to do before it's "functional":

• [ ] Make sure notebooks run
• [ ] Add the test server warning back, preferably in a reusable way as was before.

Otherwise, most of the files have been converted.

That said, the documentation really needs a rewrite. I refrained from changing any content as much as I could, with rare exceptions. But the main problems, to me, are:

• Information is all over the place. A few cohesive examples with a proper ToC would be better (I think).
• The formatting is bad.
• There is a mix of styles. Some examples even talk about "exercises" (e.g., here).
• Some of the content is outdated.

How should we move forward with this PR? I don't mind rewriting (some of) this documentation, but I think it would be better to merge this at one point so it's easier to collaborate. WDYT? @eddiebergman @LennartPurucker