Added the Unity User Manual (#4955)

* Added the Unity Manual

Updated free-courses-en.md with the official free manual from Unity

* Update free-courses-en.md
Co-authored-by: NSeth <seth@elypia.org>
Co-authored-by: NSeth <seth@elypia.org>

free-courses-en.md

@@ -235,6 +235,7 @@
 
 * [CS50 2019 - Games Track](https://www.youtube.com/playlist?list=PLhQjrBD2T382mHvZB-hSYWvoLzYQzT_Pb) - David J. Malan (Harvard OpenCourseWare)
 * [Unity Beginner Fundamentals](https://learn.unity.com/course/unity-beginner-fundamentals)(Unity Learn)
+* [Unity User Manual](https://docs.unity3d.com/Manual/)
 
 
 ### Git