Update free-programming-playgrounds.md (#5946)

* Update free-programming-playgrounds.md

Added a few C++ playgrounds to the list

* Update free-programming-playgrounds.md

Placed the links in alphabetical order. I mistakenly didn't do this in my initial PR.

more/free-programming-playgrounds.md

@@ -52,6 +52,8 @@
 
 ### <a name="cpp"></a>C++
 
+* [LabStack](https://code.labstack.com/cpp)
+* [OnlineGDB](https://www.onlinegdb.com/online_c++_compiler)
 * [SoloLearn](https://code.sololearn.com/cpp)