Add "associated type" to glossary (#2331)

# Why
For example, while reading [this
line](3269cb9e4c/src/std-traits/operators.md (L39)),
a user may want to quickly look up what an "associated type" is.

This also helps with translations. In Vietnamese, there isn't a good
translation for this term, so keeping the original term and providing a
description of it in the glossary would be very helpful.

src/glossary.md

@@ -32,6 +32,9 @@ h1#glossary ~ ul > li:first-line {
   Dynamic memory allocation on [the heap](memory-management/review.md).
 - argument:\
   Information that is passed into a function or method.
+- associated type:\
+  A type associated with a specific trait. Useful for defining the relationship
+  between types.
 - Bare-metal Rust:\
   Low-level Rust development, often deployed to a system without an operating
   system. See [Bare-metal Rust](bare-metal.md).