Stroke-maps

New data for ambulance catchment areas.

Now includes the Small Area file.

This is an edited version of the 50K file from OpenDataNI. The polygons have been simplified to reduce the file size by a third.

Simplification info:
+ Use the mapshaper website to simplify polygons. [https://mapshaper.org/](https://mapshaper.org/)
+ Default Visvalingam / weighted area setting
+ Saved a copy simplified to 20% for the `stroke-maps` data and a copy simplified to 0.5% for the streamlit apps.
+ Resulting file sizes 31.0MB and 3.5MB respectively

Northern Ireland data added; big improvements to the docs and examples for github-pages.

Separated from the stroke pathway module.