Auto merge of #4904 - alexcrichton:clean-docs, r=alexcrichton

Auto merge of #4904 - alexcrichton:clean-docs, r=alexcrichton

Delete the old docs, lift up the new

This commit deletes the old documentation now that the "official source" is the
main book. The book is now lifted up directly into `src/doc` instead of
`src/doc/book`.

The CI no longer builds documentation and has been updated to just run mdbook to
make sure there's no errors. The documentation will actually get published in
the rust-lang/rust repo