Linking to ancillary material formatted as Jupyter nb?
I think you're asking the wrong question here.

For the sake of persistent archiving you should put all ancillary material at a sustainably archived website specifically designed to preserve scholarly communications material e.g. Zenodo https://zenodo.org/ or Dryad  http://datadryad.org/ or Figshare https://figshare.com/

Refer-to or cite the ancillary material archived on Zenodo with the Zenodo DOI that is generated upon archival of the material. From within that ancillary material which is archived, you can link to an nbviewer prettified instance of your Jupyter nb.

Direct-linking to nbviewer from within your article is a really bad idea. In five or ten years time it's very likely that the link will be broken. Always refer to ancillary material with persistent identifiers e.g. DOIs.