Consider switching to MkDocs for documentation

[tcbrindle]

We're currently using Sphinx for our documentation, but it seems like MkDocs -- specifically Material for MkDocs -- is now preferred by many projects.

It certainly does generate very nice, modern looking documentation. Using Markdown by default (rather than RST) would possibly make things easier for new contributors (although we could in principle use Markdown for Sphinx as well). We'd also be able to move away from Sphinx's (sometimes finicky) C++ parser, which might result in more natural (C++-y) looking declarations.

C++ projects using MkDocs include mp-units and {fmt}