"The Rust Programming Language"

"The Rust Programming Language"

This pulls all of our long-form documentation into a single document,
nicknamed "the book" and formally titled "The Rust Programming
Language."

A few things motivated this change:

* People knew of The Guide, but not the individual Guides. This merges
  them together, helping discoverability.
* You can get all of Rust's longform documentation in one place, which
  is nice.
* We now have rustbook in-tree, which can generate this kind of
  documentation. While its style is basic, the general idea is much
  better: a table of contents on the left-hand side.
* Rather than a almost 10,000-line guide.md, there are now smaller files
  per section.