nixos/postgresql: add section about pg_config

See https://discourse.nixos.org/t/i-cannot-for-the-life-of-me-find-the-package-that-has-pg-config/66244/4

I decided against doing this in its own nixpkgs manual: the line
to draw is quite blurry already (e.g. we have documented our package
removal policy in here as well) and having to check two manuals for a
single subsystem feels pretty annoying to me.

The relevant part - where to find pg_config - is written at the top. I
decided to give a bit more context about the way our packaging works
since I realized a few times now that I don't remember all the details
about the problems we had in the past and having to look up individual
commit messages for that isn't very productive.

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