Add a "notation" section to the glossary of the book

[jackh726]

While we talk about bound vars, inference vars, and placeholders in different parts of the book and sometimes introduce notation, we should have a single consolidated section in the glossary to specifically mention some notation, such as:

?0 - Inference Var
^0, ^1.0 - Bound var
!0 - Placeholder
A :- B - A if B

Maybe others

[hayashi-stl]

Could notation be its own appendix?

[camelid]

@rustbot claim