How to setup YubiKey SSH authentication for Synology NAS under DSM 7: advanced users only

Synology DSM 7 supports YubiKey logins to the Synology NAS console. Because SSH access results in an equivalent, if not greater, security exposure, it makes sense to lock that door with the same level of security (it would be silly to have YubiKey secured console login but password secured SSH access with port 22 for…