Document `community.postgresql` collection dependencie

[Davido264]

I had issues on a minimal installation of ansible due to not having the community.postgresql collection installed. I think the dependency on this collection should be documented.

[github-actions]

This issue has been marked 'stale' due to lack of recent activity. If there is no further activity, the issue will be closed in another 30 days. Thank you for your contribution!

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