Document nftables/nft

nftables is replacing iptables nowadays and we should document it. I don't know if we have to keep the section about iptables or if it should be replaced. It probably depends on how good the nftables ecosystem is (does fwbuilder support nft as output backend?).

I got a contribution from Howard Johnson in this PDF. It will likely help you to get this going. Don't forget to credit him in your commit message if that's the case.

(In my private mail archive, Message-Id was cc9505ce-0d93-e1b7-64ca-2d0c4b458419@BridgeportContractor.com)