API doc: document Paper endpoint (#1427)
* de-emphasize `metrics` as a repo type (kinda dead)
* add the most important paper endpoint imo
* emphasize this
* Update docs/hub/api.md
Co-authored-by: Omar Sanseviero <osanseviero@gmail.com>
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Co-authored-by: Omar Sanseviero <osanseviero@gmail.com>
