Hand over GHC management to ghcup from stack

The new user using stack with default settings will typically end up with a duplicated development environment, as stack will try to pull GHC versions whenever it cannot find a version required by a project that is already installed from GHCup.

This notice helps avoiding the issue. One might argue that this does not belong to ghcup's documentation, but this very docs already hints at such a notice (https://www.haskell.org/ghcup/about/#why-should-i-use-ghcup-over-stack).