SPARK-917 Improve API links in nav bar

docs/_layouts/global.html

@@ -74,12 +74,12 @@
                         <li class="dropdown">
                             <a href="#" class="dropdown-toggle" data-toggle="dropdown">API Docs<b class="caret"></b></a>
                             <ul class="dropdown-menu">
-                                <li><a href="api/core/index.html">Spark Core for Java/Scala</a></li>
+                                <li><a href="api/core/index.html#org.apache.spark.package">Spark Core for Java/Scala</a></li>
                                 <li><a href="api/pyspark/index.html">Spark Core for Python</a></li>
                                 <li class="divider"></li>
-                                <li><a href="api/streaming/index.html">Spark Streaming</a></li>
-                                <li><a href="api/mllib/index.html">MLlib (Machine Learning)</a></li>
-                                <li><a href="api/bagel/index.html">Bagel (Pregel on Spark)</a></li>
+                                <li><a href="api/streaming/index.html#org.apache.spark.streaming.package">Spark Streaming</a></li>
+                                <li><a href="api/mllib/index.html#org.apache.spark.mllib.package">MLlib (Machine Learning)</a></li>
+                                <li><a href="api/bagel/index.html#org.apache.spark.bagel.package">Bagel (Pregel on Spark)</a></li>
                             </ul>
                         </li>