_content/security: add policy.html

We want to use go.dev/security as a landing page for all things Go
security.

In order to do so, go.dev/security now redirects to
go.dev/security/policy.

Once the landing page is created and all references to go.dev/security
are changed, go.dev/security will be its own page, with a link to the
security policy.

Change-Id: I281c38d3322abca35c87d9655debe126cc84cd64
Reviewed-on: https://go-review.googlesource.com/c/website/+/398255
Reviewed-by: Roland Shoemaker <roland@golang.org>
Reviewed-by: Zvonimir Pavlinovic <zpavlinovic@google.com>
diff --git a/_content/security.md b/_content/security.md
new file mode 100644
index 0000000..2c6d1a4
--- /dev/null
+++ b/_content/security.md
@@ -0,0 +1,3 @@
+<!--{
+	"Redirect": "/security/policy"
+}-->
diff --git a/_content/security.html b/_content/security/policy.html
similarity index 100%
rename from _content/security.html
rename to _content/security/policy.html