docs: Add recommended Postgres configuration

Created by: efritz

This PR adds a bit of documentation around our recommended settings for Postgres. Fixes https://github.com/sourcegraph/sourcegraph/issues/20403.

These numbers were derived from an enterprise customer that was running into major scaling issues. These numbers are not far off from what Cloud SQL generated for us by default (except they're just now parameterized based on container resources). I believe these to be a good set of recommendations for medium to large instances (super large instances will of course need to be tuned individually).

I'd like to hear from both @sourcegraph/cse and @sourcegraph/distribution teams on this PR as a reaction to the general effort (and whether we should or reformat, rephrase, relocate). I think that we should adopt as many of these settings in our distribution configs (k8s config maps and volume mounted postgres.conf for docker-compose) along with comments, but the source of truth should be here as we provide a discussion around the settings and can link to troubleshooting docs as we fill them out.