docs feature request: describe a simple GitHub Actions setup

Preamble. ghcup has become a primary tool to obtain various versions of the Haskell toolchain, so much so it's sometimes advised to bypass the haskell/setup GitHub action and use ghcup directly (among other things, this allows adding the pre-release channel, which haskell/setup action has been failing to add support for for several months now).

Proposal. But the natural question is: where do I start if I want a ghcup-backed GHA setup? I think it'd be helpfull to give (a reference to) a simple example of such setup in the User Guide. Mind you, the guide already has the Continuous Integration section, which would be a natural home for such an example.

Threats to validity. GitHub is not the only platform around there. Still, it's the one used by many novices, so following the 20/80 rule, I think GHA is worth serving in the guide leaving other platforms for community to research.