Docs examples 03/14: Rework dropdown combobox examples

Part 3 of 14 in the stack that splits up the approved draft !6295 (closed) into reviewable pieces. The content is exactly what was reviewed there; I've only sliced it.

Related to #3575.

Pages: dropdown combobox (GlCollapsibleListbox). This page is big enough to be its own MR: the listbox examples become a role picker, a labels picker, a reviewers picker, branch/tag groups, an environment picker with debounced search, and an assignee picker, each placed in the section that documents its feature, with unique titles.

Safe to publish without the MRs after it.