Add CustomField resource policies and tests

- Add policies block with HasPermission for read/create/update/destroy
- Add authorizers: [Ash.Policy.Authorizer] to CustomField resource
- Add custom_field_policies_test.exs (read all roles, write admin only)
- Fix CustomField path in roles-and-permissions doc (lib/membership)

lib/membership/custom_field.ex

@@ -50,7 +50,8 @@ defmodule Mv.Membership.CustomField do
   """
   use Ash.Resource,
     domain: Mv.Membership,
-    data_layer: AshPostgres.DataLayer
+    data_layer: AshPostgres.DataLayer,
+    authorizers: [Ash.Policy.Authorizer]
 
   postgres do
     table "custom_fields"
@@ -79,6 +80,13 @@ defmodule Mv.Membership.CustomField do
     end
   end
 
+  policies do
+    policy action_type([:read, :create, :update, :destroy]) do
+      description "Check permissions from user's role"
+      authorize_if Mv.Authorization.Checks.HasPermission
+    end
+  end
+
   attributes do
     uuid_primary_key :id