Add basic GATs reference information

[jackh726]

There really isn't a ton to add here. Almost all of the syntax related to GATs falls out from existing syntax.

[jackh726]

Does anything about where clauses like, e.g., Self: 'a, need to be added here? I'm thinking about this specifically: https://rust-lang.github.io/generic-associated-types-initiative/explainer/required_bounds.html

I can try to add a short explanation