cmd/golangorg: allow frames from self, for /doc/codewalk/*

Necessary for serving /doc/codewalk/* on go.dev.

Change-Id: I16d7eef08468cc92bcfcd57104329424e41a68a8
Reviewed-on: https://go-review.googlesource.com/c/website/+/362957
Trust: Russ Cox <rsc@golang.org>
Reviewed-by: Jamal Carvalho <jamal@golang.org>
Website-Publish: Russ Cox <rsc@golang.org>

cmd/golangorg/csp.go

@@ -103,6 +103,6 @@ var csp = map[string][]string{
 		"tagmanager.google.com",
 	},
 	"frame-ancestors": {
-		none,
+		self,
 	},
 }