Similar to a Clippy Book, which describes how to use Clippy, a book about how to
contribute to Clippy might be helpful for new and existing contributors. There's
already the doc directory in the Clippy repo, this can be turned into a
mdbook.

#6011

Steps to completion:

• Write a Clippy book explaining its features and how to use it https://doc.rust-lang.org/nightly/clippy/
• Convert the contributor documentation to a book (partly done)
• Update dev part of book with feedback from contributors
• Add book to docs.rs drop down