Auto merge of #4457 - behnam:book-pub, r=alexcrichton

[doc] Publish book to gh-pages

Export mdBook into `target/doc/book/`, to make it accessible at <http://doc.crates.io/book/>

I have tested the changes on my repo (but when I had this commit on top of the other PR) locally. The result is at <http://code.behnam.es/rust-cargo/book/>.

Tracker: <https://github.com/rust-lang/cargo/issues/4040>