PR shows how to fix the guides problem and tidies the repo up significantly

[gavinsimpson]

Main changes are tweaks to the the first figure with the scale/legend issue, showing how to stop it changing to be a legend - use guide_colourbar().

Also this PR cleans up the r folder a lot - there isn't any reason to use the static dump of gratia and the dev version from github / r-universe works just fine.

I've also indicated some better coding practises - not using hard coded paths, having a folder for generated figures, etc