Rollup merge of #30836 - Luke-Nukem:master, r=steveklabnik

Ref issue [30825](https://github.com/rust-lang/rust/issues/30825)

This commit should suffice to add a concise introduction to the concept of crates.
My only worry, is that it is maybe too concise; but, the book seems to be written with the understanding that the new Rust user is coming from another language, and so will understand what a Library or Code Package is.