Ideas for new tasks/changes to the existing tasks

Just a collection of some ideas to improve the course and make some tasks better. A few things I have on my mind and discussed with @vrom911:

- [ ] Chapter 1: A simpler task before `lastDigit` that doesn't require thinking about corner cases.
- [ ] Chapter 2: Maybe eta-reduction task where you shouldn't eta-reduce?
- [ ] Chapter 3: A task on typeclasses to write the type of a function like `foo x y = show (x + y)`.
- [x] Chapter 3: Make sure that the first fight is only one round, no recursion.
- [ ] Chapter 3: Clarify, that in `Append` people don't need to create a separate `List` newtype for lists.
- [ ] Chapter 4: The advanced task can make use of Monads to practice this topic more.

No files or tests are named. Start by turning the checklist into agreed, scoped changes for Chapters 1–4 with maintainers, then update the corresponding course tasks and confirm each selected item is addressed.

Written by the indexing model from the issue text.