host rustdocs for librustc and so forth on forge

[nikomatsakis]

It would be great to have the rustdocs for the compiler's crates hosted on forge to help with onboarding new contributors.

[mgattozzi]

As I mentioned before I have a branch here that works and it's just a matter of tweaking travis to have the right tools to actually be able to build the compiler so it can make the docs. It should work after that's done. Then we can setup a cron job or something to run the master branch daily to update the docs.

[est31]

cc @Manishearth

[steveklabnik]

This is a duplicate of #29893