cmd/golangorg: fix security redirect loop

Fixes golang/go#64672.

Change-Id: I5355a1dc9993ef0471c4787f4964e45519a6f6dc
Reviewed-on: https://go-review.googlesource.com/c/website/+/549215
Reviewed-by: Michael Pratt <mpratt@google.com>
Auto-Submit: Russ Cox <rsc@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

_content/doc/security.md

@@ -1,3 +0,0 @@
-<!--{
-	"Redirect": "/security"
-}-->

cmd/golangorg/testdata/godev.txt

@@ -100,3 +100,12 @@ redirect == /play/
 
 GET https://go.dev/play/p/
 redirect == /play/
+
+GET https://go.dev/security
+redirect == /security/
+
+GET https://go.dev/security/
+redirect == /doc/security/
+
+GET https://go.dev/doc/security/
+body contains Security