[Side Effects] Better explanation slides & example

[csalv22]

TODO:

• The sample for the useEffect in the explanatory slides is quite complex. Idea: Reuse the same example as in the official react beta docs (with chatroom). It is a lot simpler to understand (even though a little bit abstract).
• The custom hooks slides feel a bit empty. In the sample with the custom hook, there is no hook logic inside. Add some state and potential useCallback there as well as some calculated property (i.e. usePagination?).