replica: add postgres configuration files into long-running branch

[kevinwojo]

It appears that 3.39 did not include postgres configuration files in the "(changes unrelated to upgrade)" commit.

On branch 3.39-customer-replica
Your branch is up to date with 'origin/3.39-customer-replica'.

Untracked files:
  (use "git add <file>..." to include in what will be committed)
	codeinsights-db/
	codeintel-db/
	pgsql/

Context: https://github.com/sourcegraph/customer/issues/854#issuecomment-1109916993

Since new customer-replica branches are based on the previous version's branch, this will be a fix-forward.

Checklist

• Sister deploy-sourcegraph change:
• All images have a valid tag and SHA256 sum

Test plan

Manually tested, this branch is also more for documentation.

[caugustus-sourcegraph]

This PR seems incomplete - doesn't it also require the changes made to the docker volume mounts?