Docs menu link should link to introduction.

layouts/header.html

@@ -43,7 +43,7 @@
         <div class="collapse navbar-collapse" id="navbar">
           <ul class="nav navbar-nav navbar-right main-nav">
             <li><a href="/">Overview</a></li>
-            <li><a href="/docs">Documentation</a></li>
+            <li><a href="/docs/introduction/overview">Documentation</a></li>
             <li><a href="/community">Community</a></li>
             <li><a href="https://github.com/prometheus"><i class="fa fa-github"></i> Github</a></li>
           </ul>