doc: add link to App Engine docs to front page

R=golang-dev, gri
CC=golang-dev
https://golang.org/cl/4539100

doc/all.css

@@ -198,3 +198,8 @@ span.alert {
   padding: 0.25em 1em;
   background: #F4F4F4;
 }
+sup.new {
+  color: red;
+  font-size: 8px;
+  line-height: 0;
+}

doc/root.html

@@ -78,6 +78,7 @@ google.setOnLoadCallback(loadFeed);
             <li><a href="/doc/effective_go.html">Effective Go</a></li>
             <li><a href="/doc/go_faq.html">Go FAQ</a></li>
             <li><a href="/doc/docs.html">Other Documentation</a></li>
+            <li><a href="http://code.google.com/appengine/docs/go/">Go for Google App Engine</a> <sup class="new">New!</sup></li>
           </ul>
         </div>
         <div id="resources-contributors" >