machines/acheron: add gg23 router replacing styx
Intel N355 mini PC, 2x I226-V, UEFI/NVMe disko, systemd-boot. Only
2configs + retiolum + gg23 for now; NIC naming matches styx (et0 uplink,
int0 LAN). Retiolum card in retiolum/ (10.243.11.2), server tag.