Fold prep.js functionality into rustdoc

[brson]

The standalone docs in doc/ are built by preprocessing them with prep.js then running through pandoc. Eliminate prep.js and just use rustdoc for all our documentation.

[brson]

Depends on #11393

[huonw]

#11393 is closed, and I'm working on this (by making rustdoc accept a plain markdown file as input).