Add an example of creating a choropleth map

[JWCook]

A choropleth map of observations would be a great example to add. This can be just for a single country for now, preferably at the county (or equivalent) level.

Example: https://altair-viz.github.io/gallery/choropleth.html

Ideally this would use standardized place IDs (like FIPS codes in the United States) instead of iNat place IDs, to make it easier to combine this with other geospatial datasets. Plan B would be to just use iNat place IDs and polygons.