chore: Add documentation for using the Postgres FDW with AuthZed and SpiceDB

Author: josephschorr

app/authzed/guides/_meta.ts

@@ -1,4 +1,5 @@
 export default {
   "picking-a-product": "Picking a Product",
   cloud: "Getting Started with Authzed Cloud",
+  "postgres-fdw": "Using Postgres FDW with AuthZed",
 };