Add an example notebook for plotting CMIP6 data using cartopy

[jacnugent]

Description

This notebook will add an additional example showing how to combine rooki and intake-esgf to plot regridded/subsetted CMIP6 data with cartopy.

Notes

This notebook will be part of the Pythia Cook-off 2024; the plan may change.

[mgrover1]

Nice work! Closing this out with #31