Add a glossary to the Rust Reference

[FreeFull]

Currently, it's pretty hard to find information about specific constructs such as type Foo = Bar; in the Reference unless one already knows in which section, and under which subsection it would be. A glossary of keywords, attributes and other relevant things at the end of the Reference would be massively helpful in finding the information.

[emberian]

We have this information in the wiki as the Rusticon. It doesn't see many edits these days, maybe it should live in tree.

[FreeFull]

I suppose what I was thinking about would be more of an index rather than a glossary, but having both be in the reference would probably be a good idea. It's not easy to find the Rusticon unless one knows it exists.

[steveklabnik]

I'm going to consider this as part of #16676 , thanks.