root: rework and expand security policy

Signed-off-by: Jens Langhammer <jens.langhammer@beryju.org>

# Conflicts:
#	SECURITY.md

website/docs/security/policy.mdx

@@ -0,0 +1,5 @@
+# Security Policy
+
+import SecurityPolicy from "../../../SECURITY.md";
+
+<SecurityPolicy />

website/sidebars.js

@@ -281,5 +281,15 @@ module.exports = {
                 "troubleshooting/missing_admin_group",
             ],
         },
+        {
+            type: "category",
+            label: "Security",
+            link: {
+                type: "generated-index",
+                title: "Security",
+                slug: "security",
+            },
+            items: ["security/policy"],
+        },
     ],
 };