Find and reference new chatroom in docs

[sshine]

Since the nixos.org Matrix has disabled registration, it is not possible for newcomers to join it.

This makes it less suitable as a channel to ask Terranix questions in.

This issue exists as a placeholder to discover another chatroom.

[sshine]

The old Matrix server is retiring as of today, so the need for a new official chatroom became that much more interesting. I'll try and see if we can gather some more niche Nix communities around a chatroom so it will be more likely to actually find anyone.