docs: start of updating 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... :/