doc/designs: add description of identity mapping in IPA
    
    Fixes: https://pagure.io/freeipa/issue/9477
    
    Signed-off-by: Alexander Bokovoy <abokovoy@redhat.com>
    Reviewed-By: Florence Blanc-Renaud <flo@redhat.com>
    Reviewed-By: Rob Crittenden <rcritten@redhat.com>