Listbox: create developer guide and example for grouped options

[ianmcburnie]

Let's re-explore grouped options, similar to: https://www.w3.org/WAI/ARIA/apg/patterns/listbox/examples/listbox-grouped/

This is a preparatory step before we can get to a listbox with columns of options (all still navigated with UP/DOWN arrow keys).

[ednihs-yahska]

I have a draft PR for this issue.
#136
Thanks