I spend quite a bit of time helping folks over in the networking area of the Apple Support Communities. If I can help get you started here with your IPv6 questions, related to your AirPort Extreme base station, ask away. But as @jfb has mentioned you can find a lot of helpful folks over at the ASC.
How to configure your Pi-hole with the AirPort Extreme base station is actually shown in one of the images jfb has already provided. You would enter your Pi-hole's IPv4 address as the first "DNS Servers" and its IPv6 address in the "IPv6 DNS Servers" slot. The key is to have already assigned static local IP addresses for your Pi-hole. You do NOT want your Pi-hole to have a dynamically assigned IP address.