[Tutorialcpp] Feedback

Hi again, I'v got another bunch of feedbacks (a total of more than 50 already) They are very happy, but one aspect showed up multiple times: They'd like to get some explanations after a multiple-choice quizz, why the answers are right or wrong. I guess the only way to do this is to add another explanation after the quizz, which is in a collapsible block. Cheers H