Remove incorrect role="tabpanel" from navs docs

role="tabpanel" is not appropriate as role for the overall container - it only applies to the individual content parts of each tab

Closes https://github.com/twbs/bootstrap/issues/22284

(note that there are still some fundamental problems with the tabs in both docs and their actual JS - this is only a quick fix to an obvious markup error in the docs)

added accessibility docs v4 labels

changed milestone to %v4.0.0-beta

closed