Add section quizzes to Hello Nexflow and Nextflow Run

[vdauwera]

Use @ewels's quiz package (as in #694) to add per-section quizzes to the intro courses (plan to add some to other courses later).

Goal is to provide intermediate opportunities for learners to revise and solidify their knowledge on key points. Will serve as preview of what might be in the Training Week quizzes as well.