docs: update docs to use MyST

[pfackeldey]

This PR updates the docs to:

• use MyST markdown instead of restructured text (except for API ref; they're a bit fragile right now so I kept them as they are)
• use a modern theme (pydata)
• reduce ~200 bugs/typos from existing doc strings
• add a User Guide section that can be filled with more 'hands-on' docs

[pfackeldey]

The CI doesn't seem to pickup the Makefile from the PR to build the docs... :/

[lgray]

I'll poke at this later today or tomorrow to see if I can get RTD consuming it correctly.

[lgray]

@pfackeldey OK - it's picking up things correctly now! Have fun :-)

[pfackeldey]

Hi @ikrommyd and @lgray,
I've added quite some content to the docs (with the help of Codex :P). Afterwards I went through manually and adjusted/fixed obvious hallucinations or things that are non-sense, and added some stuff by hand. I do not intend to update the example notebooks with this PR.
Please have a look how you like the docs and the content. Let me know if you want something in addition or different. In principle I would consider this PR done (for now).

[ikrommyd]

Thanks a ton @pfackeldey! I will take a look as soon as I can.