ZOOKEEPER-4356: Code blocks do not render correctly in ZK docs site

* Configure to use FENCED_CODE_BLOCKS.
* Note: TABLES is the default setting. See https://github.com/walokra/markdown-page-generator-plugin for details on default `pegdownExtensions` setting

Author: Nihal Jain <nihaljain.cs@gmail.com>

Reviewers: maoling <maoling@apache.org>, Damien Diederen <ddiederen@apache.org>

Closes #1741 from NihalJain/ZOOKEEPER-4356

zookeeper-docs/pom.xml

@@ -49,6 +49,7 @@
           <headerHtmlFile>${project.basedir}/src/main/resources/markdown/html/header.html</headerHtmlFile>
           <footerHtmlFile>${project.basedir}/src/main/resources/markdown/html/footer.html</footerHtmlFile>
           <copyDirectories>images,skin</copyDirectories>
+          <pegdownExtensions>TABLES,FENCED_CODE_BLOCKS</pegdownExtensions>
         </configuration>
       </plugin>
       <plugin>