[FLINK-6952] [docs] Add link to Javadocs

docs/_includes/sidenav.html

@@ -126,6 +126,7 @@ level is determined by 'nav-pos'.
   {% endif %}
 {% endfor %}
   <li class="divider"></li>
+  <li><a href="https://ci.apache.org/projects/flink/flink-docs-release-{{site.version_short}}/api/java"><i class="fa fa-external-link title" aria-hidden="true"></i> Javadocs</a></li>
   <li><a href="http://flink.apache.org"><i class="fa fa-external-link title" aria-hidden="true"></i> Project Page</a></li>
 </ul>