Review extrinsic metadata specification

https://docs.softwareheritage.org/devel/swh-storage/extrinsic-metadata-specification.html?highlight=extrinsic%20metadata

Following up the discussion in swh/devel/swh-deposit#3677 (closed): We should see if the extrinsic metadata specification needs refactoring following the decision taken on the metadata-only deposit.

At the end of this discussion in this task we should:

capture the conclusion in the docs
if needed, open a task to refactor the ERMDS

Migrated from T3681 (view on Phabricator)

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