Add new interactive resource about CSS (#3874)

free-programming-interactive-tutorials-en.md

@@ -116,6 +116,7 @@
 * [CSS Diner](http://flukeout.github.io)
 * [Flexbox Defense](http://flexboxdefense.com)
 * [Flexbox Froggy](http://flexboxfroggy.com)
+* [Grid Garden](https://cssgridgarden.com)
 * [Learn by doing beginner projects](https://dash.generalassemb.ly)
 * [Learn HTML & CSS interactively](https://www.codecademy.com/learn/web)
 * [Prototyping a professional website](https://www.codecademy.com/learn/make-a-website)