Docs: Switch to using MyST Markdown

[mbercx]

Fixes #924

Convert the entire documentation in MyST Markdown, which is easier to use due to its similarity with the ubiquitous Markdown and cleaner syntax.

Moving to MyST Markdown also makes it easier/more consistent to use MyST-NB and eventually move to fully executable documentation.

Some remaining RestructuredText elements:

• The autodoc generation of the API. There is also https://sphinx-autodoc2.readthedocs.io/en/latest/index.html. I remember there was a reason I didn't use it, but I don't remember why. ^^
• Automated generation of CLI documantation.
• aiida.sphinxext items

[sphuber]

Thanks @mbercx . I'm fine with the move in principle. Just noticed that the styling is quite different in the new build. Is this because some styling rules are not migrated properly yet, or because one of the updated dependencies changes the style? Did sphinx-book-theme change the appearance that much? I think I prefer the current style personally

[mbercx]

I think the styling changes are mainly due to the bump in the sphinx-book-theme indeed, which was necessary. We can change the defaults for sure, though, just let me know which changes you're not a fan of and I can look into it.