Setup dependencies

Currently we request: https://github.com/carpentries-incubator/lesson-R-packaging/blob/36c4cbd6d8c981413b7d2205c3513ba9083b2085/learners/setup.md#L30

However, lots of the instructions involve `usethis`. This happens to work because it's a dependency of `devtools`, but I would rather it be something we explicitly ask users to install in case this changes, and so they get into the good habit of installing the packages they are using.

In any case, `roxygen2` is also a dependency of `devtools` so if we wanted the minimal set we would remove that.

Start at learners/setup.md around line 30 and review the package installation instructions against the packages used throughout the lesson. Confirm whether usethis should be explicitly installed and whether roxygen2 belongs in the minimal set; done means the setup instructions clearly list the agreed dependencies.

Written by the indexing model from the issue text.