Graphics in concept documents

In #490, there is a long discussion about adding plots to the Randomness concept's `about.md`, to illustrate visually how the various functions work with statistical distributions. This seems like a good idea: R is substantially about data visualization in its real-world use, and ggplot2 is an outstanding and popular plotting package.

_However_, I entirely lack graphics skills, the accessibility guidelines are important but challenging to meet, and supporting dark mode is proving painful.

To avoid delaying syllabus launch, I am proposing to make graphics a post-launch task, to sort out separately for a version 2 concept.

I put some stuff on [Github pages](https://colinleach.github.io/supplementary_docs/statdist-r.html) to show the technical content (with default formatting). There's a `.graphics` subdirectory in #490, with work-in-progress to make something website-compliant. We'll probably merge that PR in the next few days.