DOIs for SIGMathLing Resources

@dginev writes on #7 (closed):

I am reading about getting a DOI for the dataset, but it is a bit more complicated given our circumstances. Zenodo may provide a DOI, but we are 1) above their standard 50GB quota, so would need special permission - though this is minor, but 2) I believe they only host publicly available data, so using Zenodo means we can't restrict access to SIGMathLing members. Which means it is likely a no-go for now.

The second option for issuing a DOI is to become a CrossRef member, but that has a slightly salty price tag - $275/year for the smallest plan. We could partner with an existing member institution, but that implies we would need to host the data under their domain, instead of our own kwarc.info.>

We should be going ahead without DOIs for the moment, so spinning this off into a separate issue.