Updated _Sidebar (markdown)

_Sidebar.md

@@ -5,6 +5,7 @@
 * [raylib data structures](raylib-data-structures)
 * [raylib external dependencies](raylib-dependencies)
 * [raylib platforms and graphics](raylib-platforms-and-graphics)
+* [raylib input system](raylib-input-system)
 * [raylib internal buffers](raylib-internal-buffers-(2D-vs-3D))
 * [raylib custom shaders](raylib-generic-uber-shader-and-custom-shaders)
 * [raylib coding conventions](raylib-coding-conventions)