Document CLI option to select TLS client certificate #678

The Cyberduck CLI gains a --certificate <alias> option to select the TLS client certificate from the system certificate store (iterate-ch/cyberduck#18553, discussion iterate-ch/cyberduck#18551).

The CLI documentation at https://docs.duck.sh/cli/ needs a section describing the option, including that the certificate is only used for the TLS handshake and login still requires a username.