v5: Make navbar active links consistent

Two sets of changes:

• Updates the navbar docs to put .active on .nav-link instead of .nav-item like the rest of our navs. Fixes #30652 (closed).

• Removes two selectors, .active > .nav-link and .nav-link.show, because as far as I can tell, those are duplicative and unused, respectively. .active just needs to be on the .nav-link, and .show is always on the parent .nav-item because it toggles the .dropdown-menu.

Am I missing something with those two selectors? Clicking around on dropdowns in the nav shows no issues thus far.

Regarding consistency, I also just noticed that .btns get .btn.show for the dropdown-menu though. Do we consolidate and make sure .show is always on the .btn and .nav-link instead of parents? Punted to #30846.