Position: sticky is included in Edge 16

https://crbug.com/781344

Closes #23319. Both `.text-muted` and `.btn-outline-secondary` (and indeed all secondary items) make use of `$gray-600`. New value provides a contrast ratio of 4.69.

Closes #24341

Closes #25067

Explicitly re-adds the "hand" `cursor:pointer` for non-disabled `.btn`, `.close`, `.navbar-toggler` elements, as well as forcing `page-link` pagination controls to always have the "hand" cursor (even if an author uses it on, say, `<button>` elements for a dynamically updating in-page pagination).

Controversial, as everybody jumped on the bandwagon following this article https://medium.com/simple-human/buttons-shouldnt-have-a-hand-cursor-b11e99ca374b - which does have its merits of course, but there are also counter-arguments like http://kizu.ru/en/issues/cursor-pointer/

And seeing the amount of issues we've seen following the change, and the potential complexity needed to consistently address the cursor issue (see https://github.com/twbs/bootstrap/issues/24156 where i explore how the cursor needs to be based on "intent", and how that's tough to determine), I'm favouring an opinionated take here of just reintroducing the `cursor:pointer`.

- Changes the wrapping label to a div so we can style the label instead of another element while also supporting form validation.
- Fixes form validation styles for custom file input (closes #24831).
- Updates docs with validation styles (also adding example feedback text while I was there) and new how it works section.