Add a documentation tab.

It's not there, but well a reminder to self that we'll actually need to write some documentation.

_includes/header.html

@@ -16,6 +16,7 @@
       <div class="trigger">
         <a class="page-link" href="{{ site.baseurl }}/">Home</a>
         <a class="page-link" href="{{ site.baseurl }}/installation/">Installation</a>
+        <a class="page-link" href="{{ site.baseurl }}/documentation/">Documentation</a>
         <a class="page-link" href="{{ site.baseurl }}/about/">About</a>
       </div>
     </nav>