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/5.1/getting-started/rfs/index.html

@@ -68,6 +68,10 @@
 </div>
 
 
+<div class="d-block px-3 py-2 text-center text-bold skippy">
+  <a href="https://getbootstrap.com/" class="text-white text-decoration-none">There's a newer version of Bootstrap!</a>
+</div>
+
     <header class="navbar navbar-expand-md navbar-dark bd-navbar">
   <nav class="container-xxl flex-wrap flex-md-nowrap" aria-label="Main navigation">
     <a class="navbar-brand p-0 me-2" href="/" aria-label="Bootstrap">