[datasets] update gnomAD datasets available via Datasets API/Annotation DB

[pwc2]

Not all of the Hail Tables and MatrixTables that are publicly available on the gnomAD downloads page are currently available in the Datasets API/Annotation DB.

This PR makes the following changes to the datasets available via the Hail Datasets API/Annotation DB:

• Add gnomad_genome_sites Table, versions: 3.1.1, 3.1.2
• Add gnomad_hgdp_1kg_subset_dense MatrixTable, version: 3.1.2
• Rename gnomad_hgdp_1kg_callset MatrixTable to gnomad_hgdp_1kg_subset_dense, version: 3.1
• Add gnomad_hgdp_1kg_subset_sparse MatrixTable, version: 3.1.2
• Add gnomad_hgdp_1kg_subset_sample_metadata Table, version: 3.1.2
• Add gnomad_hgdp_1kg_subset_variant_annotations Table, version: 3.1.2
• Add gnomad_variant_co-occurrence Table, version: 2.1.1
• Add gnomad_pca_variant_loadings Table, versions: 2.1, 3.1

Other general changes:

• Add/update the schema .rst files, for the datasets listed above, for the docs
• Set the example dataset loaded in hl.experimental.load_dataset to be the most recent gnomad_hgdp_1kg_subset_dense MatrixTable (version 3.1.2)

[danking]

I spot-checked a few of them, they all looked good. One comment about updating to at least the most recent major version for our example schema.

updated schemas

[danking]

Thank you much, this is a great improvement!