Continuous documentation server for rust packages

[brson]

Similar to #1453, we can use rustpkg + rustdoc to automatically generate all documentation for all rust libraries and display them on a website. This will require a somewhat-central registry of blessed-for-CI packages (as we're no longer operating on the assumption of a central package list for the tool)

[nikomatsakis]

Still true that this is theoretically possible.

[emberian]

This is going to be impossible unless you want to keep built copies of all dependencies for all the crates around, as rustdoc_ng runs resolve and typeck to get proper hyperlinking in documentation.

[huonw]

Haskell's Hackage would seem to have the same problem (of running type-checking, and requiring built dependencies), and it seems to manage; reopening.

[emberian]

#8125

The rustdoc_ng work enabling this is done, just need infrastructure now.

[steveklabnik]

Triage bump: closing in favor of rust-lang/crates.io#91