Add a Community Projects page

This adds a new page to the docs listing community projects -- those created outside of Apache Spark that are of interest to the community of Spark users.   Anybody can add to it just by submitting a PR.

There was a discussion thread about alternatives:
* Creating a Github organization for Spark projects -  we could not find any sponsors for this, and it would be difficult to organize since many folks just create repos in their company organization or personal accounts
* Apache has some place for storing community projects, but it was deemed difficult to work with, and again would be some permissions issues -- not everyone could update it.

Author: Evan Chan <[email protected]>

Closes #2219 from velvia/community-projects-page and squashes the following commits:

7316822 [Evan Chan] Point to Spark wiki: supplemental projects page
613b021 [Evan Chan] Add a few more projects
a85eaaf [Evan Chan] Add a Community Projects page

docs/_layouts/global.html

@@ -111,6 +111,7 @@
                                 <li class="divider"></li>
                                 <li><a href="building-spark.html">Building Spark</a></li>
                                 <li><a href="https://cwiki.apache.org/confluence/display/SPARK/Contributing+to+Spark">Contributing to Spark</a></li>
+                                <li><a href="https://cwiki.apache.org/confluence/display/SPARK/Supplemental+Spark+Projects">Supplemental Projects</a></li>
                             </ul>
                         </li>
                     </ul>
@@ -151,7 +152,7 @@ <h1 class="title">{{ page.title }}</h1>
                     MathJax.Hub.Config({
                         tex2jax: {
                             inlineMath: [ ["$", "$"], ["\\\\(","\\\\)"] ],
-                            displayMath: [ ["$$","$$"], ["\\[", "\\]"] ], 
+                            displayMath: [ ["$$","$$"], ["\\[", "\\]"] ],
                             processEscapes: true,
                             skipTags: ['script', 'noscript', 'style', 'textarea', 'pre']
                         }

docs/index.md

@@ -107,6 +107,7 @@ options for deployment:
   * [OpenStack Swift](storage-openstack-swift.html)
 * [Building Spark](building-spark.html): build Spark using the Maven system
 * [Contributing to Spark](https://cwiki.apache.org/confluence/display/SPARK/Contributing+to+Spark)
+* [Supplemental Projects](https://cwiki.apache.org/confluence/display/SPARK/Supplemental+Spark+Projects): related third party Spark projects
 
 **External Resources:**