nixos/postgresql: improve docs on how to upgrade
nixos/postgresql: improve docs on how to upgrade

* It's IMHO a slight overkill to deploy an additional container even if
  it's never supposed to be running. Also, the currently suggested
  approach wouldn't use the default state-directory for the new version.
* Explain the structure of the state-directories and where the
  version-numbers are actually coming from.
* Mention `./analyze_new_cluster.sh` & `./delete_old_cluster.sh`.

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