Provide a recommended set of iptables/nftables rules to help in case of DoS attacks

A bunch of DoS attacks are essentially ongoing since June 2022 and we discussed a bunch of potential solution to improve things for our users. One thing folks started to experiment with is trying to come up with good iptables rules to help fighting ongoing attacks.

This ticket is for collecting all the information we gathered so far and coming up with some rules we can recommend to our relay operators (and updating our support guidelines accordingly).