[pagex] implement visibility flags

[voidcannon] migrate to visibility flags

content/static/404/404.pagex.js

@@ -1,5 +1,5 @@
 import {
-    VoidcannonContentType
+    VoidcannonContentType, VoidcannonVisibilityFlags
 } from "voidcannon";
 
 import NotFoundFragment from "voidcannon/src/fragments/notfound/NotFound.fragment.js";
@@ -14,5 +14,8 @@ export default {
     static: {
         fragment: NotFoundFragment,
         params: {}
-    }
+    },
+    visibility_flags: [
+        VoidcannonVisibilityFlags.HIDDEN_IN_SITEMAP
+    ]
 }