create a nice landing web page for exported dataset

Our list of exported datasets is rather bare-bone: https://annex.softwareheritage.org/public/dataset/

We should make it more appealing, including references to papers and consistent description of all datasets.

That might require switching from a simple dir listing to a static site generator backed by a directory listing (ikiwiki?).

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Migrated from T1743 (view on Phabricator)