Ch4: add example do files

[mariaruth]

suggest to add for both power calc simulations and randomization inference. This is not urgent, but would be quite useful.

[kbjarkefur]

We had that previoulsy but we removed it after the first review of chapter 4.

[mariaruth]

Previously had do-files? remind me why we took them out? i looked back at issues and didn't see one on this but maybe i missed it.

[kbjarkefur]

Because they were too complex. Random inference is best coded if we define a program, and that takes too much space to explain. I think it was something similar for power calc.

[mariaruth]

got it. perhaps add to the wiki then, and link here? this feels incomplete without any link to how to do it, but i definitely see this could get overly complex for the book.