This commit introduces a new example to the Altair documentation showcasing  choropleth maps faceted by category.

The example visualizes the distribution of suitable habitat for different species across US counties, using the proposed new Species Habitat dataset from vega-datasets (vega/vega-datasets#684).

Key features of this example:

- Demonstrates the use of `alt.Chart.mark_geoshape()` for geographical visualizations.
- Shows how to create faceted maps for comparing categorical data across geographic regions.
- Utilizes the `transform_lookup` and `transform_calculate` transforms for data manipulation within Altair.
- Uses a CSV data file temporarily hosted in the vega-datasets repository branch (pending dataset merge).

This example addresses issue #1711, which requested a faceted map example for the Altair documentation.

Co-authored-by: Mattijn van Hoek <mattijn@gmail.com>