Add Higher-Kinded Types section to Scala 2 Tour

Author: Dev-CodeBox

_overviews/higher-kinded-types.md

@@ -0,0 +1,61 @@
+---
+layout: multipage-overview
+title: Advanced Type Features
+partof: scaladoc
+overview-name: Scaladoc
+num: 4
+permalink: /overviews/scaladoc/advanced-type-features.html
+---
+
+# Advanced Type Features in Scala
+
+This section introduces some advanced type system features in Scala, including **Higher-Kinded Types (HKT)**, type bounds, and type projections.
+
+## Higher-Kinded Types
+
+Higher-Kinded Types allow abstracting over type constructors. For example:
+
+```scala
+trait Functor[F[_]] {
+  def map[A, B](fa: F[A])(f: A => B): F[B]
+}
+```
+
+Here `F[_]` is a type constructor that takes a single type parameter.
+
+### Example Usage:
+
+```scala
+val optionFunctor = new Functor[Option] {
+  def map[A, B](fa: Option[A])(f: A => B): Option[B] = fa.map(f)
+}
+
+val result = optionFunctor.map(Some(2))(_ * 2) // Some(4)
+```
+
+## Type Bounds
+
+Scala supports upper and lower bounds:
+
+```scala
+def max[T <: Ordered[T]](x: T, y: T): T = if (x > y) x else y
+```
+
+Here `T <: Ordered[T]` means `T` must implement `Ordered`.
+
+## Type Projections
+
+You can refer to a type member of another type:
+
+```scala
+class Outer {
+  class Inner
+}
+
+val o = new Outer
+val i: o.Inner = new o.Inner
+```
+
+## Summary
+
+These features allow Scala developers to write highly abstract and reusable code. Higher-Kinded Types, type bounds, and type projections are cornerstones of Scala’s expressive type system.