Feature 4 dataset metadata distributions

[mbjones]

Modifications of the Dataset.md guide for feature #4 on metadata distribution info. See the issue #4 for details of the design and discussion.

[mbjones]

Main changes for this PR can be reviewed as rendered (with the diagram): https://github.com/ESIPFed/science-on-schema.org/blob/feature_4_dataset_metadata_distributions/guides/Dataset.md#metadata

The ADR for this PR from @ashepherd is: https://github.com/ESIPFed/science-on-schema.org/blob/feature_4_dataset_metadata_distributions/decisions/4-dataset-metadata-distributions.md

[smrgeoinfo]

the schema.org "Values expected to be one of these types" for subjectOf are CreativeWork or Event, but the structured data testing tool doesn't throw an error for @type download, so its not breaking.

ADR looks good to me

[datadavev]

Suggest revising to acknowledge equivalence of subjectOf and about. It seems unnecessarily restrictive to emphasize the subjectOf approach, and may impact adoption of the recommendation. For example, ro-crate suggests the about approach.