Use pygments to syntax highlight rustdocs

[brson]

Pandoc has support for pygments and we have a rust plugin for pygments, so we should make them work together

[brson]

Pandoc doesn't support pygments after all - its default theme is the pygments theme, but it uses kate syntax definitions

[bstrie]

Wouldn't it be possible to support highlighting in the library docs by using whatever mechanism provides highlighting in the tutorial?

[brson]

Yes. The reason I haven't wanted to do that is because we use a node.js script to postprocess the tutorial and I don't want to introduce that dependency to rustdoc. Possibly we could use the same script to do the syntax highlighting on the client side.

[thestinger]

Closing in favour of #5259 since pandoc now has Rust syntax highlighting support.