Documentation headings should be linkable

[huonw]

E.g. I wanted to link to the dev_dependencies section of http://crates.io/manifest.html on #523 but couldn't do it directly.

(This would be fixed by using rustdoc to generate the docs. :P )

[steveklabnik]

Big 👍 here

[alexcrichton]

I like the idea of using rustdoc. I have a branch to do this, but it is dependent on rust-lang/rust#17090 to be usable.

[alexcrichton]

cc #550