Document how to set up secure connections between HA peers in versions before 1.9.5
https://kea.readthedocs.io/en/latest/arm/agent.html?highlight=Secure%20connections#secure-connections-version-before-1-9-5 documents how to secure the connection to a single CA and only works if you have an HTTPS-knowledgeable client. For HA, where all involved entities i.e. CAs don't speak HTTPS, it's a bit more complex and involves changes to both ngins and Kea configuration too. Let's document that.