Use PyData Sphinx Theme

[raphaelvallat]

See raphaelvallat/pingouin#432

[remrama]

@raphaelvallat I got a good start on this. Whereas this PR is technically just about converting to the new pydata-theme, I think we both agreed that a decent documentation overhaul was also necessary. Would you rather have me (A) submit a PR strictly for the theme change and then a subsequent PR for the content, or (B) submit both in the same PR?

(A) has a cleaner workflow but (B) is probably easier for me as the writer and you as the reviewer, since we wouldn't have to look around twice. I'm catching lots of stuff as I modify the theme.

I'm thinking about things like fixing typos, moving the notebooks into a Tutorial page (and probably put Quickstart as a first page of Tutorials), adding an Installation page or section, etc... I'm happy to take some liberty on those decisions and let you check them out in a review, so I understand if you prefer approach (A) so that you could identify the changes more easily. Or maybe a broad overview in (B) is okay.

[raphaelvallat]

@remrama B is fine since it will make everything easier on your side! Please do take liberty in the decisions, I'll let you know if I really disagree with something during review, but I doubt it'll happen :) The update of the documentation was long overdue anyway. Thanks for working on this 🙌