Add documentation bundle to nightly build

[gavinb]

It would be incredibly useful to be able to download a bundle of all the documentation provided at:

http://doc.rust-lang.org/index.html

This content is presumably built as part of the nightly build process anyway, so it would suffice to zip it up (or create a .tar.gz) and add a download link.

Since dustup.sh seems to be the recommended way to install nightly builds, perhaps there could be a flag such as --with-docs to also download the corresponding doc bundle.

As someone who frequently works offline, having access to the docs (especially the library references!) without having to rebuild everything would be a great help. Thanks!

[brson]

Dupe of #14916.