Scala 2 language tour should have a "higher-kinded types" section

[SethTisue]

as previously discussed on #850.

I agree with @martijnhoekstra who wrote:

HKTs are a feature that really sets scala apart from other languages like kotlin, an should IMO therefore also be part of the tour, though maybe a bit towards the end

and

It's one of the features that really sets Scala apart from the competition.

[muskan3006]

@SethTisue I will try to take a look at this