Suggest GHCup instead of Haskell Platform

[konsumlamm]

The What you need to dive in section in the introduction currently recommends downloading the Haskell Platform. However, the Haskell Platform is deprecated and it is recommended to use GHCup instead, so the section should be updated.

[smith558]

Definitely! I myself use GHCup for my environment management too as most people probably do. I will look into updating this when I find a moment. Alternatively, feel free to open a pull request. 🙂