Add Sandcastle Custom DataSource example

This adds a `Custom DataSource` example to sandcastle.  Currently it's a standalone example without an accompanying web-site tutorial; but eventually it will have a more detailed website counterpart.

I also renamed the GeoJSON files to have a geojson extension to prevent confusion.
This could use a comment explaining how we are setting the value of `show` on all the polylines that are part of the given series to make them be displayed.