Rollup merge of #57212 - phansch:improve_rustc_book_contributing, r=steveklabnik
Rollup merge of #57212 - phansch:improve_rustc_book_contributing, r=steveklabnik

docs(rustc): Link to the book's source in rustc

This makes the source of [the rustc book](https://doc.rust-lang.org/rustc/what-is-rustc.html) book a bit more discoverable.

aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust,aidancully/rust,aidancully/rust,graydon/rust,graydon/rust,graydon/rust