Wiki: Extend quiz questions

[ljacqu]

• GitHub now shows an overview of the titles on the right-hand side. "Quiz questions" is a top-level title semantically but I had used a lower level to make the title smaller. This now looks weird in the overview -> use top-level titles for quiz questions
• Add more quiz questions and actually provide answers to them (the idea from the beginning was to actually provide the answers, albeit in a collapsible panel). Hide answers with collapsed sections: https://docs.github.com/en/get-started/writing-on-github/getting-started-with-writing-and-formatting-on-github/quickstart-for-writing-on-github#adding-a-collapsed-section