Documentation extension: Port forwarding configuration: recommend to use DHCP reservation to make sure nextbox LAN IP is constant

[jerabaul29]

On the page mentioning port forwarding configuration: https://docs.nitrokey.com/nextbox/remote/port-forwarding.html . It may be a good idea to mention that the user needs to make sure that the forwarding is 'stably pointing to the nextbox'.

Many routers will allow to forward a web facing port to a LAN IP and port. As the nextbox uses DHCP, there is no guarantee that its LAN IP will remain constant. A way around that is to enable DHCP reservation on the router, so that the nextbox (recognized by its MAC address) always gets the same IP on the LAN.

Another option is to give a static IP address to the nextbox, but that may be a bit more demanding (change dhpcp.conf, make sure the IP is outside the router DHCP range, etc, + what happens if the nextbox gets moved to be behind another router etc).