(Re?)enable LaTeX math in documentation

[SimonSapin]

http://static.rust-lang.org/doc/master/rust.html#machine-types currently has text like $[0, 2^8 - 1]$. It looks like it’s supposed to be rendered as a nicely formatted equation, and maybe was at some point.

[huonw]

AFAICT neither sundown nor hoedown (#12776) support LaTeX directly (unlike Pandoc). One solution is using MathJax to render it on the client.

[adrientetar]

Well if it's just for pows, we don't need MathJax but we can rather just add <sup> in the Markdown source instead.
There's a regression of the removal of rustdoc exposed here through is that we don't process references anymore (@huonw): ^[this is a reference].
With pandoc they would have arrows with a link to the end of the page but it's not part of standard Markdown and so I don't think it's built-in Sundown/Hoedown.
Supporting it in our wrapper would be cool but removing it shouldn't hurt I think.

[huonw]

Yes, that's a work-around. Although, there are places where I would like proper rendering (example).

I'm sure other people will wish to use rustdoc with rendered mathematics, so it would still be neat to at least have --include-in-header mathjax-header.template and --include-after-body mathjax-after.template that inserts the contents of those files into the final output, or even a --mathjax flag (or something) that does the mathjax insertion automatically, although that might be regarded as too specialised.

[Hoverbear]

One thing to note using markdown and MathJAX is that underscores need to be escaped from _ to \_ when you compile.