Disabled link cleanup

per https://www.w3.org/TR/html-aria/#docconformance

It is NOT RECOMMENDED to use aria-disabled="true" on an a element with an href attribute.

NOTE If a link needs to be "disabled", remove the href attribute.

This PR removes the unnecessary href="#", tabindex="-1", and aria-disabled="true" from disabled links in both docs pages and examples. aria-disabled="true" is kept for disabled link-based buttons (that have role="button") as there it's appropriate to use (you want to convey to assistive technologies that this thing you're claiming is a button is also disabled at the moment)

Further, the PR extends the "Link functionality caveat" to show the "proper" way (removing href and adding .disabled class only) to disable a link, but then explains what to do if that's not possible (and then keeps an example with all the traditional href="#" tabindex="-1" aria-disabled="true", but explains clearly that it's not ideal). Same sort of explanation is also added to the pointer event utilities page

x-ref https://github.com/twbs/bootstrap/pull/34874#issuecomment-911769998