Add high-level check list for GeoZarr/STAC data producers

[j08lue]

The documentation in this project includes all the specs for producing and cataloguing Sentinel data in GeoZarr.

To ease the onboarding of external teams to this process, we should add a high-level checklist somewhere that they can start from and then dive deeper as needed.

[d-v-b]

I am naive to stac but shouldn't we have a to_stac_json method on our data structures?

[j08lue]

Prior art on ETL into cloud-native formats + STAC: https://github.com/stac-utils/stactools