Add vignette for geographies

## Describe the solution you'd like
We need a good vignette to explain the subtleties / possibilities around selecting geographies using eesyapi:
- How to select a simple combination of geographies
- How to select all geographies at a given level within another geography (e.g. all regions in England, all LAs within the North East)

I'm thinking this should be linked appropriately from the workflow vignette.

## Additional context
Linked equivalents for time periods (#46) and filter items (#45).