[ WIP ] - User Story understanding the XArray Genotype Call Dataset

[jerowe]

SGKit has (at least one) xarray datasets to simplify API creation for variant calling datasets.

I'm creating a documentation PR in order to understand the xarray dataset created by sgkit.api.create_genotype_call_dataset .

This will be broken down into 3 parts:

1. Minimal example using just straight numpy arrays with pictures.
2. Example reading in from a VCF. This will be with the understanding that this is for conceptual understanding only, and vcfs should be converted using the vcf_to_zarr function.
3. Full example using the Malaria Phase 2 datasets already preprocessed to zarr.