Add 4.x to index

Author: wilsonge

index.html

@@ -207,6 +207,7 @@ <h1>Joomla! API Documentation</h1>
                                     <div class="thumbnail"><img src="https://api.joomla.org/images/cms.png">
                                         <div class="caption">
                                             <p>
+                                                <a class="btn btn-large btn-block" href="https://api.joomla.org/cms-4/index.html">CMS 4.x</a>
                                                 <a class="btn btn-large btn-block" href="https://api.joomla.org/cms-3/index.html">CMS 3.x</a>
                                                 <a class="btn btn-large btn-block" href="https://api.joomla.org/cms-2.5/index.html">CMS 2.5</a>
                                             </p>