Add Kotlin Quick Reference online book. (#6103)

* Add Kotlin Quick Reference online book.

https://kotlin-quick-reference.com/

* Update free-programming-books-langs.md

* Update books/free-programming-books-langs.md
Co-authored-by: NDavid Ordás <3125580+davorpa@users.noreply.github.com>

* Update free-programming-books-langs.md
Co-authored-by: NDavid Ordás <3125580+davorpa@users.noreply.github.com>

books/free-programming-books-langs.md

@@ -1321,6 +1321,7 @@ That section got so big, we decided to split it into its own file, the [BY SUBJE
 
 * [Kotlin Notes for Professionals](https://goalkicker.com/KotlinBook/) - Compiled from StackOverflow documentation (PDF)
 * [Kotlin Official Documentation](https://kotlinlang.org/docs/reference/)
+* [Kotlin Quick Reference](https://kotlin-quick-reference.com) - Alvin Alexander (gitbook)
 * [Learning Kotlin](https://riptutorial.com/Download/kotlin.pdf) (PDF)