Generate RDoc documentation.

This is currently still very messy and we probably want to work out the
best way to declare what parts of our DSL/what files are "public".

Still, even if this is a WIP I'd rather get something committed sooner
rather than later and start iterating on this as a replacement for
`example_formula.rb` and the formula cookbook.

To test:
```bash
cd $(brew --prefix)/Library/Homebrew && \
  rdoc formula.rb requirement.rb utils.rb &&\
  open doc/index.html
```

Closes #32470.