Add instructions for using sockets to connect to DB

[icelander]

trafficstars

Arch Wiki has instructions on setting up a socket-based connection. We should create an official document on how to use sockets for database connections.

[justinegeffen]

Thanks, @icelander. I don't see instructions in the install docs for TCP or Unix sockets, but I do see instructions for WebSockets: https://docs.mattermost.com/install/installing-ubuntu-2004-LTS.html so wondering if this request still applies. Thank you!