New "What's Here" for Ruby Hash RDoc

As I wrote last week, I've been working on the Ruby documentation. This work includes 20 or so "What's Here" sections, for some of the more important classes and modules. Each is a collection of categorized 1-line descriptions of the methods.

Here's the one for Hash, which is new in 3.1: https://docs.ruby-lang.org/en/3.1/Hash.html#class-Hash-label-What-27s+Here.