Ship "the rustdoc book"

[carols10cents]

Located here: https://github.com/rust-lang/rust/tree/master/src/doc/rustdoc/src

This book documents Rustdoc and all its options. To do:

• Write the doc tests chapter
• Write the in-source directives chapter
• Write the passes chapter
• Write the plugins chapter
• Create an "environment variables" chapter
• write rustbuild integration to generate the book
• link the book from the doc index page

It's no use, Mr. James – it's turtles docs all the way down.

Originally filed as rust-lang/book#725:

[steveklabnik]

Moving to p-high and assigning myself; i'd like to get this resolved in the next release cycle. If anyone else wants to pitch in, please let me know!