Add examples to GeoZarr spec repository

[maxrjones]

This issue is to track an action item from the GeoZarr meeting today to add a simple script that generates GeoZarr compliant Zarr stores to the GeoZarr spec repository. This could build off zarr-developers/geozarr-spec#42 and https://github.com/briannapagan/geozarr-validator/. The script should include a demonstration/explanation of grid mapping, an example for Web Mercator TMS, and a custom TMS for COG-like downsampling.

I think that @vincentsarago was interested in tackling this issue, but let me know if that's not the case. I'm glad to help if you don't have time or would like to pair.