Move datasets used on documentation cite under the Kandy project

https://kotlin.github.io/kandy/simple-usa-states-polygons-geojson.html

For example:
```
val usaStates =
GeoDataFrame.readGeoJson("https://raw.githubusercontent.com/AndreiKingsley/datasets/refs/heads/main/USA.json")

usaStates.plot {
geoPolygon()
}
```

Start with the linked simple-usa-states-polygons-geojson.html documentation example and inspect how its USA.json dataset is referenced. Identify where documentation datasets can live under the Kandy project, then update the example to use that location; done when the example no longer depends on the external datasets repository.

Written by the indexing model from the issue text.