graph dataset: redirect from annex page to doc

this page: https://annex.softwareheritage.org/public/dataset/graph/ should become a redirect to the doc page: https://docs.softwareheritage.org/devel/swh-dataset/graph/dataset.html

to avoid giving the false impression that the latest dataset is the one pointing to an old version

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Migrated from T4468 (view on Phabricator)