doc: update install.html for binary distros, add install-source.html

R=bsiegert, rsc, r
CC=golang-dev
https://golang.org/cl/5605047

doc/gccgo_install.html

 <!--{
-	"Title": "Setting up and using gccgo"
+	"Title": "Setting up and using gccgo",
+	"Path": "/install/gccgo/"
 }-->
 
 <p>