doc/designs: add encrypted DNS design documents
    
    Add design page for Encrypted DNS traffic support.
    
    Related: https://pagure.io/freeipa/issue/9605
    Signed-off-by: Francisco Trivino <ftrivino@redhat.com>
    Reviewed-By: Antonio Torres <antorres@redhat.com>
    Reviewed-By: Rob Crittenden <rcritten@redhat.com>