FUSE design doc: dangling links to data model, SWHIDs, etc.

Links from the published version of the design document to pages from other packages are not rendered properly. See e.g.: The Software Heritage {ref}data model <data-model> is a Direct Acyclic Graph in that page.

They should be expanded properly by Sphinx thanks to MyST, which was added in swh/devel/swh-docs!78 (closed). And //they were// expanded properly for a while. Something broke since.

Migrated from T2785 (view on Phabricator)

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