doc: link to references from docs page

Fixes #3667.

R=golang-dev, iant
CC=golang-dev
https://golang.org/cl/6492078

doc/docs.html

@@ -56,6 +56,9 @@ A must read for any new Go programmer. It augments the tour and
 the language specification, both of which should be read first.
 </p>
 
+<h3 id="ref"><a href="/ref/">Go References</a></h3>
+<p>Language specification, memory model, and detailed documentation for the commands and packages.</p>
+
 <h3 id="appengine"><a href="https://developers.google.com/appengine/docs/go/gettingstarted/">Getting Started with Go on App Engine</a></h3>
 <p>
 How to develop and deploy a simple Go project with