Link page source from the index footer

To make it easy for others to find the source code for the site, add
a URL to the cgit tree for it in the page footer.

Change-Id: Ie1d88e4aa6e4a572dacfa5b6e4d79536c920ab32

www/index.html

@@ -174,7 +174,7 @@
           <!-- Copyright -->
             <div class="copyright">
               <ul class="menu">
-                <li>&copy; Zuul Contributors. Licensed under Creative Commons Attribution 3.0</li><li>Title image by James E. Blair</li><li>Design based on Arcana from: <a href="http://html5up.net">HTML5 UP</a></li>
+                <li>&copy; Zuul Contributors. Licensed under Creative Commons Attribution 3.0</li><li>Title image by James E. Blair</li><li>Design based on Arcana from: <a href="http://html5up.net">HTML5 UP</a></li><li><a href="https://git.openstack.org/cgit/openstack-infra/zuul-website/tree/">Browse page source</a></li>
               </ul>
             </div>