Add old elastic docs experience warning

Author: reakaleek

extra/docs_landing.html

@@ -416,8 +416,20 @@
     margin: 0 auto 50px;
     border: none;
   }
+
+  .old-elastic-experience-warning {
+    padding: 1em;
+    margin-bottom: 1em;
+    border: 1px solid #ccc;
+    background: #FFFFD2;
+    color: black;
+  }
 </style>
 
+<div class="old-elastic-experience-warning">
+  You are viewing the old Elastic docs experience. To view the new experience, go to <a href="https://www.elastic.co/docs">elastic.co/docs</a>.
+</div>
+
 <div class="legalnotice"></div>
 
 <div id="subnavigation" class="subnavigation">