Example for geopandas polygon geometry: Use the Natural Earth dataset, change to Africa

[yvonnefroehlich]

Description of proposed changes

Use the Use the Natural Earth dataset in the example for geopandas polygon geometry. For details see #4223 (comment).
Let's hope the request / download is more stable and this will make our docs build less often fail 🚀.

Have to choose a region (and adjust the positionn of the colorbar):

• Africa (we have this already in the paper)
• South America
• Europe (we have this already for the rivers)
• or any other suggestion people like

Chicago (currently)	Africa	South America	Europe

Related to

• Example for geopandas line geometry: Use Natural Earth dataset, change to South America #4229
• Which data to use for geopandas.GeoDataFrame choropleth example? #2786
• Add gallery example showing usage of polygon objects from a geopandas.GeoDataFrame (choropleth map) #2796
• Add gallery example for plotting geopandas.GeoDataFrame objects #1374

Fixes #4223

Preview: https://pygmt-dev--4228.org.readthedocs.build/en/4228/gallery/maps/choropleth_map.html

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[michaelgrund]

I would go with Africa.

[seisman]

Looks good to me.