Host compiler documentation

[brson]

The API documentation for the compiler is useful, and only available online via @Manishearth. It should be hosted by rust-lang.

Need to decide where on the doc server they are hosted - probably separately from the end-user docs; ensure the makefiles can produce them in a separate location from the user docs; add the build rule to the dist builders; make buildbot upload them to the right place.

cc @edunham

UPDATE: Mentoring instructions available here.

[steveklabnik]

This is a dup of #28309, but gets to the heart of the issue, so I'm going to close it in favor of this one.

I've been talking with @edunham about this a few times over the last few weeks, I think the plan we discussed was a /internals path.

[edunham]

@Manishearth how are you currently generating the contents of https://github.com/Manishearth/rust-internals-docs ?

[Manishearth]

./configure --enable-compiler-docs; make docs. I was considering at some point to make it turn on private docs as well (using rustdoc --passes)