Docs: use `<button>` for dropdown/popover examples, explain preference over `<a>...

Docs: use `<button>` for dropdown/popover examples, explain preference over `<a>` for dropdowns, tweak collapse prose (#37432)

* Expand dropdown explanation for use of `<a>`, use `<button>`s for modal examples

* Remove redundant `role="button"` from collapse test `<button>` elements

* Tweak language

* Tweak collapse prose