Rearrange contents in chapter 4

[webartifex]

https://nbviewer.jupyter.org/github/webartifex/intro-to-python/blob/develop/04_iteration/03_content.ipynb

Move the part on "Indefinite Loops" to the top. Change the game logic a bit and include the continue statement already.

Consider removing the part on the map-filter-reduce paradigm entirely (the numeric example is not the same as in chapter 8 part 1 !)

Mention that in operator does exactly the shown linear search logic.

[webartifex]

New small exercise: "Calculating PI with Leibniz' formula"

cf. "Classic Computer Science Problems with Python", pp. 19