Add a "private IPv6 address range" generator to the "use IPv6 with nebula" doc

https://simpledns.plus/private-ipv6 is a website that will provide you a random `/64` in the private allocations for IPv6. It's not necessary, but it'd be sweet to add something similar as an embedded component in #223 / https://nebula.defined.net/docs/guides/upgrade-to-cert-v2-and-ipv6/ to allow users to easily generate an IPv6 network range for use in their nebula networks.