Document steps for GVS cleanup for base use case [VS-586]

[rsasch]

Documents steps to clean up GVS assets in the following use case:

• the samples in the callset are going to be joint called once (no sub-cohorts, no re-using the model)
• the outputs (VCFs, indexes, interval lists, manifest, sample name list) have all been copied to an independent location (not in the workspace bucket)
• no need to maintain provenance
• the workspace where the pipeline was run is only being used for this GVS callset

[codecov]

Codecov Report

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The diff coverage is n/a.

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[kbergin]

Actually I have one suggestion :)