The documentation surrounding TON DNS is here:
https://ton.org/docs/participate/web3/dns

Its official site is here:
https://dns.ton.org/

I must stress that this is not the same DNS as what you would find when registering a .com domain address on a typical domain registrar. There is currently no ".ton" nameserver that can be connected to the rest of the world wide web. Instead, these are essentially nicknames for addresses. This is helpful because addresses are hard to memorize, but "jeremy.ton" is easy to remember.

If you're looking for TON Proxies and TON Sites, where websites can be displayed on the TON network, then documentation surrounding it can be found here:

https://docs.ton.org/learn/services/sites-www-proxy