Docs: groups and roles/permissions architecture, Group moduledoc

- groups-architecture: normal_user and admin can manage groups.
- roles-and-permissions: matrix and MembershipFeeCycle :linked for own_data.
- group_policies_test: update moduledoc.

test/mv/membership/group_policies_test.exs

@@ -3,7 +3,7 @@ defmodule Mv.Membership.GroupPoliciesTest do
   Tests for Group resource authorization policies.
 
   Verifies that own_data, read_only, normal_user can read groups;
-  only admin can create, update, and destroy groups.
+  normal_user and admin can create, update, and destroy groups.
   """
   use Mv.DataCase, async: false