rethink how we display DOI svg badges

[tacaswell]

If you go to https://matplotlib.org/citing.html only a (random) fraction of the svg's for the DOIs load. Investigating using browser tools says we are getting 429 (TOO MANY REQUESTS) responses back from zenodo.

We probably should embed a copy of the svg in our docs (and write a helper script to do that).

[timhoffm]

Should we ask the zenodo people how to best handle this?

[QuLogic]

We could link the general one, and ask people to pick a version-specific one from there?

[timhoffm]

Should be good enough.