[book] Add a Glossary section?

[behnam]

What terms are used in Cargo and what they mean is a common theme in Cargo issues/pulls, as well as in the documentation. Recently I just updated some of the docs to use the correct term in their place. (See #4376 and #4375)

I think with the expansion of Cargo features, it makes sense to have a Glossary document that describes each term and points to the detailed description.

We can add this to the new mdBook-based docs, and not in-repo.

Here's some example terms that I have seen confusions about in the docs and public spaces, like the users forum:

• crate
• package
• build target (as in platform and its binary format)
• target (as in lib, bins, tests, examples, ...)
• crate file
• package repository (as in git repo)
• package registry (as in crates.io), a.k.a. package index
• package root (the directory)
• workspace root (the directory)
• workspace root package (a package or its manifest)
• repository root (the directory)

What do you think?

[alexcrichton]

cc @carols10cents

[behnam]

Also cc @istankovic, since this would be a new section on the new mdBook-based repo.