Setup Instructions for R on Ubuntu

The [section on installing R on Ubuntu](https://carpentries.github.io/sandpaper-docs/#r-2) points to the [Cran website's instructions](https://cran.r-project.org/bin/linux/ubuntu/). However, these direct you add the Cran repository, which only has R for the LTS releases; there's no Ubuntu 22.10 version, for example.

R is in the default Ubuntu package list, as `r-base` for Ubuntu 22.10. Unless there's a particularly strong reason to recommend the latest R, it seems like it'd be better to prioritise keeping the instructions compatible with the latest versions of Ubuntu?

Start with the “installing R on Ubuntu” section linked in the issue and review the current CRAN instructions alongside Ubuntu’s available packages. Update the documentation so its installation guidance remains usable for current Ubuntu releases, then verify the links and stated version support.

Written by the indexing model from the issue text.