broken link to data model / Merkle DAG diagram

The SVG diagram at https://docs.softwareheritage.org/devel/swh-model/data-model.html#data-model is 404, currently pointing to https://docs.softwareheritage.org/devel/swh-model/swh-model/images/swh-merkle-dag.svg .

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Migrated from T1449 (view on Phabricator)