R Dev Container: check for blurry screenshots in docs; improve optimization R code if necessary

[hturner]

This is issue 192 in the R Dev Container docs, looking for people to check for blurry screenshots in the docs and (if necessary) improve the R script used to automatically optimize images.

This does not require you to be able to build the docs, you can review the docs online and work with the image files in the docs/assets folder. You may need some git wizardry to retrieve some of the non-optimized images that were over-written, or else follow the instructions to start a codespace and work with it as described in the documentation (or collaborate with someone else who can do this) in order to take new screenshots.