HDFS-14590. [SBN Read] Add the document link to the top page. Contributed by Takanobu Asanuma.

(cherry picked from commit 48d7f00da8aab25707dc08219122541867edfa65)

hadoop-project/src/site/site.xml

@@ -80,6 +80,7 @@
       <item name="Commands Reference" href="hadoop-project-dist/hadoop-hdfs/HDFSCommands.html"/>
       <item name="NameNode HA With QJM" href="hadoop-project-dist/hadoop-hdfs/HDFSHighAvailabilityWithQJM.html"/>
       <item name="NameNode HA With NFS" href="hadoop-project-dist/hadoop-hdfs/HDFSHighAvailabilityWithNFS.html"/>
+      <item name="Observer NameNode" href="hadoop-project-dist/hadoop-hdfs/ObserverNameNode.html"/>
       <item name="Federation" href="hadoop-project-dist/hadoop-hdfs/Federation.html"/>
       <item name="ViewFs" href="hadoop-project-dist/hadoop-hdfs/ViewFs.html"/>
       <item name="Snapshots" href="hadoop-project-dist/hadoop-hdfs/HdfsSnapshots.html"/>