doc: add doc/articles/index.html

Fixes #3402.

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/5923043

doc/articles/index.html

+<!--{
+	"Title": "/doc/articles/"
+}-->
+
+<p>
+See the <a href="/doc/#articles">Documents page</a> for a complete list of Go articles.
+</p>