[Documentation] Add info in docs about how to make a production build

This was one of our users' question: if AltTester is imported in the Unity project, what do you have to do to make a production build?
Suggested answer: There is no need to remove the AltTester package entirely from the project, only the ALTTESTER Scripting Define Symbol should be deleted from the Player Settings. After that you can build normally as you would do in Unity.