Add DOI badges and links

[RichardLitt]

This would be useful for citing numpy, which in tern affects impact and funding for the entire project.

May be dependent on numpy/numpy#12946.

[AbhiRKeesara]

Hi @InessaPawson,

I've offered to help with this as part of the NumFOCUS Sustaining Open Source Series program.

I just commented on the upstream issue (numpy/numpy#12946) offering to create the .zenodo.json file once a maintainer enables the Zenodo webhook. Once that's done, I'll submit a PR here to add the DOI badges and links.

Happy to take this on!