Add Higher-Kinded Types section to Scala 2 Tour

[Dev-CodeBox]

This PR adds a new section Higher-Kinded Types to the Scala 2 Tour documentation.

It includes:

• Higher-Kinded Types (HKT) with Functor example
• Type Bounds (upper/lower) with sample code
• Type Projections with example classes

This addresses issue #1269 and provides simple explanations and examples for learners.

[eed3si9n]

Note that in scala/scala#5916 I've added some help message to repl :kind command, including the explanation of a proper type, a type constructor, and HKT. Also you can pass in -v to the :kind command to get English description of the kind in Scala 2.13:

scala> :k -v Option[Int]
Option[Int]'s kind is A
*
This is a proper type.

scala> :k -v Option
Option's kind is F[+A]
* -(+)-> *
This is a type constructor: a 1st-order-kinded type.

scala> trait ~>[-F1[_], +F2[_]] {}
trait $tilde$greater

scala> :kind -v ~>
~>'s kind is X[-F1[A1],+F2[A2]]
(* -> *) -(-)-> (* -> *) -(+)-> *
This is a type constructor that takes type constructor(s): a higher-kinded type.

[bishabosha]

why is this in the Scaladoc guide and not Tour of Scala?

[Dev-CodeBox]

Good catch, thanks! You're right, this should be part of the Tour of Scala, not Scaladoc.
I’ll move the content under _tour and update the layout accordingly.

[WojciechMazur]

The unrelated CI failures, would be fixed in #3269, please rebase onto latest main when it's merged