Recommend installing Stack via GHCup

[tomjaguarpaw]

In my opinion, the time is right for www.haskell.org to recommend GHCup as the single install tool, for the following reasons:

• It is desirable for users to have a single way to install all of the Haskell toolchain. It is confusing to recommend they use two different installers.
• GHCup has proven to be a reliable installation tool for the Haskell toolchain.
• GHCup has been able to install Stack for the last 15 months
• GHCup can now manage GHC versions for Stack
• GHCup installs both cabal-install and Stack by default, so it is not "biased" towards one or the other
• The current GHCup and Stack have been collaborating to make sure the two tools play well together

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This unlocks many follow ups, such as adding simple "getting started" or "start coding" sections, for example:

• Adding a Getting Started page #214
• Start coding section #208

[tomjaguarpaw]

Procedurally, this will require a majority vote (4 out of 7) of the Haskell.org committee. I recommend that the committee votes in favour.

[idabzo]

LGTM, first timers needs non-optional guidelines and clear instructions.

[TikhonJelvis]

Yay, progress :)