"Lemmy from Scratch" install steps don't mention PostgreSQL tuning like Docker does

[RocketDerp]

This is what Docker says:
https://github.com/LemmyNet/lemmy/blob/main/docker/docker-compose.yml#L108

We need to get that put into the Lemmy from Scratch steps.

Also, when the document is updated to Ubuntu 22.04 - consider which version of PostgreSQL should be recommended. As the "from Scratch" just goes with whatever Ubuntu provides by default (on my system 14.8, config file location: /etc/postgresql/14/main/postgresql.conf)