v4: Disabled link cleanup (#34924)

manual backport of #34886

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

site/content/docs/4.6/components/card.md

@@ -309,7 +309,7 @@ Add some navigation to a card's header (or block) with Bootstrap's [nav componen
         <a class="nav-link" href="#">Link</a>
       </li>
       <li class="nav-item">
-        <a class="nav-link disabled" href="#" tabindex="-1" aria-disabled="true">Disabled</a>
+        <a class="nav-link disabled">Disabled</a>
       </li>
     </ul>
   </div>
@@ -332,7 +332,7 @@ Add some navigation to a card's header (or block) with Bootstrap's [nav componen
         <a class="nav-link" href="#">Link</a>
       </li>
       <li class="nav-item">
-        <a class="nav-link disabled" href="#" tabindex="-1" aria-disabled="true">Disabled</a>
+        <a class="nav-link disabled">Disabled</a>
       </li>
     </ul>
   </div>