Updates to Functions material based on 2025 run

[ethanwhite]

• Move exercise 3 to after exercise 4 since we use exercise 4 for our in class
• remove () from the function name in one of the exercises (a student tried the_function_name() <- ...)
• Consider adding another you do exercise, at least for the non-tidyverse stuff
• Cut the functions inside functions piece
• Consider adding a simpler tidyverse function exercise for in class that can be broken up, but only if can tighten up earlier material to leave room