create a code handout of challenges without solutions

I just found that this action can be done with the `sandpaper::build_handout()` function.

This probably can generate the button that is at the end of the list of episodes: "Download Lesson Handout". Please, take a look at the following [lesson on R for Ecologist](https://datacarpentry.org/R-ecology-lesson/)

ref: https://carpentries.github.io/sandpaper/reference/build_handout.html