Setup via cvmfs should mention which system libraries / packages are expected to be available

This page here: https://github.com/key4hep/key4hep-doc/blob/main/docs/getting_started/setup.md should mention the packages that are also listed here: https://github.com/key4hep/key4hep-spack?tab=readme-ov-file#requirements

That would allow us to avoid issues like https://github.com/key4hep/key4hep-spack/issues/846