Add "rpc.idmapd plugin" design page
    
    A copy & paste from the old one.
    
    There's a "FIXME" about a note that I, unfortunately, have no clue what
    it's supposed to mean.
    
    Signed-off-by: Fabiano Fidêncio <fidencio@redhat.com>
    (cherry picked from commit 6d396f1b0c5c9d841e72fd96ed560a77d643af17)