Add notes about Pi-hole IPv6 and public admin panel access

[MichaIng]

As of: MichaIng/DietPi#4575

While we added loopback, LLA and ULA IPv6 address prefixes to the whitelist when blocking public access to admin panel, this does not work when GUAs (global unique addresses) are used within the LAN, which is pretty common. In this case we have no chance to differentiate between public and LAN IPs.

We can suggest to enable ULA (unique local address) assigning for the local network, if this is not the case yet. This can be done in the router, so that all LAN devices have three IPv6 addresses, one LLA, one ULA and one GUA. What needs testing is if within the LAN devices automatically and consequently use the ULAs, and whether the router also resolves local hostnames to ULAs.