godoc: support canonical Paths in HTML metadata

Redirect to the canonical path when the old path is accessed.

R=gri
CC=golang-dev
https://golang.org/cl/5536061
diff --git a/doc/docs.html b/doc/docs.html
index c0ced98..e99017f 100644
--- a/doc/docs.html
+++ b/doc/docs.html
@@ -1,5 +1,6 @@
 <!--{
-	"Title": "Documentation"
+	"Title": "Documentation",
+	"Path": "/doc/"
 }-->
 
 <div class="left-column">