Add "DBus Domains" design page
    
    A copy & paste from the old one.
    
    Although it links to a non-existent page at the moment, the page will be
    added by the very same PR containing this patch. IOW, the link is broken
    now but won't be when the PR lands.
    
    Signed-off-by: Fabiano Fidêncio <fidencio@redhat.com>