Hints files missing from R exercises

The inclusion of a `.docs/hints.md` file is officially optional, and was originally rare. As Exercism has matured, the value of these files to both students and mentors is increasingly recognized. Their inclusion is [encouraged](https://forum.exercism.org/t/concept-exercise-files/13289/7), at least for new exercises (both practice and concept).

I just did a search of `r/exercises/practice/`, and the only hints file I found was `vehicle-purchase/.docs/hints.md`. Even that is just an empty stub!

Adding more is not urgent, but would be valuable if anyone wants to attempt it. We only need a single file per exercise.

The basic style is a bulleted list containing links to relevant documentation. Probably short for a practice exercise, more detailed and step-by-step for a concept exercise (though that's another missing feature of the R track). Here is [an example](https://github.com/exercism/python/blob/main/exercises/practice/pascals-triangle/.docs/hints.md) from the Python track.

Writing Approaches documents is an alternative, but that is a _lot_ more work. See [Reverse String](https://github.com/exercism/r/tree/main/exercises/practice/reverse-string/.approaches) for an example.

Start by reviewing the existing r/exercises/practice/vehicle-purchase/.docs/hints.md stub and the linked Python pascals-triangle example. Choose an R practice exercise, then add a .docs/hints.md file with relevant documentation links in a concise bulleted list. Done means the selected exercise has a useful hints file committed in the expected location.

Written by the indexing model from the issue text.