Improve documention content, formatting, and html theme

[rgommers]

This is a thorough update of the existing docs pages. I went through everything (except "Home", which has almost no content, and "Install OpenBLAS", which may need a bit of discussion), updated all the existing content, streamlined the language, added some context and more up-to-date info as I saw fit.

It follows up on gh-4562, and together with that PR addresses gh-4166.

Main changes:

• Improve the html theme (add a dark mode, nicer admonitions, footnotes, an auto-hiding top bar)
• Fix all links, code markup, and grammar.
• "About" page: add all funding sources I know about, and missing major users like NumPy and R
• "Developer manual": improve testing and benchmarking descriptions
• "User manual": start with context about what is covered and link to Netlib/MKL docs; add more info on linking