Missing quiz questions and exercises

[DariusParvin]

Ideally each notebook would have some questions and exercises at the end to help reinforce the concepts introduced in the notebook. Most notebooks don't have them but some do, e.g. p2pkh notebook has a longer exercise with a completed solution and the transaction-level-timelock notebook has some quiz questions.

Any feedback on what kinds of exercises/questions would be useful to know.

PRs for suggestions would be very welcome!