[ZEPPELIN-3431] - Fix links, images in docs

docs/_includes/themes/zeppelin/_navigation.html

@@ -168,9 +168,9 @@
                 <li><a href="{{BASE_PATH}}/development/contribution/how_to_contribute_website.html">How to Contribute (website)</a></li>
                 <li role="separator" class="divider"></li>
                 <li class="title"><span>External Resources</span></li>
-                <li><a target="_blank" href="">Mailing List</a></li>
-                <li><a target="_blank" href="">Apache Zeppelin Wiki</a></li>
-                <li><a target="_blank" href="">Stackoverflow Questions about Zeppelin</a></li>
+                <li><a target="_blank" href="https://zeppelin.apache.org/community.html">Mailing List</a></li>
+                <li><a target="_blank" href="https://cwiki.apache.org/confluence/display/ZEPPELIN/Zeppelin+Home">Apache Zeppelin Wiki</a></li>
+                <li><a target="_blank" href="http://stackoverflow.com/questions/tagged/apache-zeppelin">Stackoverflow Questions about Zeppelin</a></li>
               </ul>
             </li>
             <li>