Add newer version 5 alert on top of v4 and v5 pages (#38410)

* Change newer version label of v3.3

* Change newer version label of v3.4

* Change newer version label of v4.0/v4.1/v4.2

* Add newer version alert for v4.3

* Add newer version alert for v4.4

* Add newer version alert for v4.5/v4.6 (using `.skippy` bg color)

* Add newer version alert for v5.0 (using `.skippy` bg color)

* Add newer version alert for v5.1 (using `.skippy` bg color)

* Add newer version alert for v5.2 (using `.skippy` bg color)

* Change some copy

- v4 gets 'Bootstrap 5 is here'
- v5 gets 'Theres a newer version of Bootstrap 5'

* Unique message: 'There\'s a newer version of Bootstrap!'

* Change HTML structure for focus visible

---------

Co-authored-by: Mark Otto <markdotto@gmail.com>

docs/3.4/getting-started/index.html

@@ -57,7 +57,7 @@
   </div>
 </a>
 
-    <a href="https://getbootstrap.com/" class="v4-tease">Looking for Bootstrap 4?</a>
+    <a href="https://getbootstrap.com/" class="v4-tease">There's a newer version of Bootstrap!</a>
 
 
     <header class="navbar navbar-static-top bs-docs-nav" id="top">