rustc-middle shows up in text without explanation

rustc-middle, if I'm not wrong first shows up here https://rustc-dev-guide.rust-lang.org/overview.html#tyty without mentioning what is it about. It would be nice to link to some part that defines it or just a brief mention there of what's about.

Maybe we can just add a note saying that `rustc_middle` is introduced in the next section https://rustc-dev-guide.rust-lang.org/compiler-src.html