Add choropleth as an example to Cookbook

[nc011]

It would be nice if we could provide an example of how to produce an afcharts based choropleth in the cookbook.

I recall a nice presentation at the GSS presentation champions of some work by the Office for Health Improvement and Disparities on choropleth colours. They had a few recommended colour palettes (including one based on the GAF blues: #092135, #12436D, #2073BC, #6BACE6, #ADD1F1 with options for supressed #F2F2F2 and no data #F9F9F9 too).

[nc011]

I guess this is related to: #5 but doesn't necessarily need a brand new function (just documentation on how to produce a choropleth, e.g., geom_sf, and add an appropriate colour palette).

[Olivia-Box-Power]

Thanks @nc011. We're hoping to get the extended colour palette added to the AF colours guidance. We'll then update afcharts to make this colour palette available and add an example to the cookbook.