Rollup merge of #40332 - steveklabnik:extract-book, r=alexcrichton

Rollup merge of #40332 - steveklabnik:extract-book, r=alexcrichton

Extract book into a submodule

Part of https://github.com/rust-lang/rust/issues/39588

We probably don't want to land this till after the beta branches on friday, but would still ❤️ a review from @alexcrichton , since I am a rustbuild noob.

This pr:

1. removes the book
2. adds it back in as a submodule
3. the submodule includes both the old book and the new book
4. it also includes an index page explaining the difference in editions
5. it also includes redirect pages for the old book URLs.
6. so we build all that stuff too.

r? @alexcrichton