network-setup: Mention Netplan, as used on cloud-images

Netplan is being used as the network configuration tool of choice on Debian's cloud images, starting with the Bookworm release.

This should be reflected in the devref and some hints should be given of how it can be used.

I was not really sure how to update the remote.ent file for popcon data, so I implemented some placeholder values. I'd appreciate some hints of how to do this properly.

Please let me know what you think about those changes.