TRPL edits: traits

Let's talk about generics first, since we use traits to bound them
in funtions.

Partially addresses #24325

Fixes #24271

Author: steveklabnik

src/doc/trpl/SUMMARY.md

@@ -34,10 +34,10 @@
     * [Method Syntax](method-syntax.md)
     * [Vectors](vectors.md)
     * [Strings](strings.md)
+    * [Generics](generics.md)
     * [Traits](traits.md)
     * [Operators and Overloading](operators-and-overloading.md)
     * [Drop](drop.md)
-    * [Generics](generics.md)
     * [if let](if-let.md)
     * [Trait Objects](trait-objects.md)
     * [Closures](closures.md)