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).

docs/guide.md

@@ -376,6 +376,15 @@ Examples:
 5. you can even compile ghc to an isolated location.
     - `ghcup compile ghc -j 4 -v 9.0.1 -b 8.10.5 -i /home/username/my/dir/ghc`
 
+## Handing over GHC management to ghcup
+
+It might be convenient to let ghup handle all things regarding compiler versions. Otherwise you might end up with duplicated directories, when `stack` would install ghc versions it needs for a project but cannot find on `PATH`.
+
+To prevent `stack` from installing GHC versions while also forcing it to use ghc versions managed by ghcup, add to your `stack`'s global `config.yaml`:
+
+    install-ghc: false
+    system-ghc: true
+
 ## Continuous integration
 
 On Windows, GHCup can be installed automatically on a CI runner