Move to sphinx with md plugin

[BalzaniEdoardo]

Add more flexibility to the website styling by moving to sphinx PyData theme.

Take advantage of MyST parser to keep the tutorial as scripts.

[billbrod]

To be clear, in order to handle the tutorials as we're doing (plain-text documents that get converted to notebooks), we need not just myst, but myst-nb. What we want then is to write notebooks in pure markdown. Note that this will require a change from how we're handling them now: mkdocs-gallery (like sphinx-gallery) has you write notebooks as py files, whereas myst-nb has you write them as md files. It's very possible that jupytext can convert between these formats for us; it definitely can convert between ipynb and the myst-nb markdown formats, so at worst we can convert the mkdocs-gallery py file to ipynb, and then from that to myst-nb markdown. Just so we're not having to rewrite things manually.