Reference links for learning track

Base R has been around for a long time, so there are lots of learning materials from various sources (of variable quality).
It's not entirely clear what to link to in our syllabus docs: a contrast to Julia, where `docs.julialang.org` is definitive.

Things are easier for anyone using RStudio, which has a help tab that can search all installed packages for reference documentation.

The key source is [CRAN](https://cran.r-project.org/), which contains a huge amount of stuff - both software and documentation. It's also quite confusing, so I need to get better at navigating around the site.

I've used a lot of links to the [manual](https://stat.ethz.ch/R-manual/R-devel/library/base/html/00Index.html) hosted by ETHZ (a Swiss Federal University in Zurich). It's complete, quite ugly (a nostalgia trip for those of us who used VT131 terminals linked to a VAX in the mid-80's), and has no search facility as far as I can see. A few other universities have mirrors, but ETHZ seems to be the big one.

[RDocumentation](https://www.rdocumentation.org/) seems to use the same source, but with more visual styling (good) and adverts (bad).

All the Tidyverse packages come with excellent [documentation](https://tidyverse.org/packages/), created in the last few years. That's the easy case.

There are a few online books. [Introduction to R](https://intro2r.com/) can be useful, alongside the [CRAN intro](https://cran.r-project.org/doc/manuals/r-release/R-intro.html) of the same name.

[Hadley Wickham](https://hadley.nz/) is the guiding force for modern R, authoring several of the most popular packages as well as several of the most popular textbooks. Rather wonderfully, some of his books have dual-track publishing, with O'Reilly selling the print version and a free online copy on his website. These two are useful to us:
- [R for Date Science](https://r4ds.hadley.nz/) is fairly introductory, but a large tome with a lot of content.
- [Advanced R](https://adv-r.hadley.nz/) contains stuff that is hard to find elsewhere, useful if we need to get into details.

I have print copies of both of those, and highly recommend them.

I'm sure there are other sources that I'm missing, so I expect this will be an evolving thread.

The immediate question (in PR #413) is whether [RDocumentation](https://www.rdocumentation.org/) is a preferred source. Exercism guidelines just say we should avoid any site requiring registration (there are thousands of those for R). Personally, I'm allergic to adverts, but others may be allergic to web pages that would display virtually unchanged in a `bash` terminal.