Replace adminer with pgadmin4 in the docker compose example

[dmint789]

I'm new to using Postgres, and when I was looking at the docs for the Docker image on Dockerhub, I was confused by the inclusion of the adminer image in the docker compose example, as it doesn't clarify that this is an admin tool for Postgres. I was further confused when I went to the adminer image on Dockerhub and saw that it's no longer maintained.

It would be best to replace adminer with pgadmin4 (if that is indeed the new preferred standard admin tool to use) and to clarify what the second image is for in the docs.

[tianon]

Unfortunately, pgadmin4 doesn't have an image that's part of the program, so we should probably remove the example Compose file instead. 😞

Thanks for the callout!