ZOOKEEPER-4370: add zookeeperOracleQuorums.html link to webpage index

- more details in the [ZOOKEEPER-4370](https://issues.apache.org/jira/browse/ZOOKEEPER-4370)

Author: maoling <maoling@apache.org>

Reviewers: maoling <maoling@apache.org>

Closes #1748 from maoling/ZOOKEEPER-4370

zookeeper-docs/src/main/resources/markdown/html/header.html

@@ -102,6 +102,9 @@ document.write("Last Published: " + document.lastModified);
             <div class="menuitem">
                 <a href="zookeeperHierarchicalQuorums.html">Hierarchical Quorums</a>
             </div>
+            <div class="menuitem">
+                <a href="zookeeperOracleQuorums.html">Oracle Quorum</a>
+            </div>
             <div class="menuitem">
                 <a href="zookeeperObservers.html">Observers Guide</a>
             </div>

zookeeper-docs/src/main/resources/markdown/index.md

@@ -48,6 +48,7 @@ archives.
     + [Quota Guide](zookeeperQuotas.html) - a guide for system administrators on Quotas in ZooKeeper.
     + [JMX](zookeeperJMX.html) - how to enable JMX in ZooKeeper
     + [Hierarchical Quorums](zookeeperHierarchicalQuorums.html) - a guide on how to use hierarchical quorums
+    + [Oracle Quorum](zookeeperOracleQuorums.html) - the introduction to Oracle Quorum increases the availability of a cluster of 2 ZooKeeper instances with a failure detector.
     + [Observers](zookeeperObservers.html) - non-voting ensemble members that easily improve ZooKeeper's scalability
     + [Dynamic Reconfiguration](zookeeperReconfig.html) - a guide on how to use dynamic reconfiguration in ZooKeeper
     + [ZooKeeper CLI](zookeeperCLI.html) - a guide on how to use the ZooKeeper command line interface