add example data set to showcase features

[maxheld83]

It would be nice to have a good example data set, which we can default to, so people can see what this can do without having to go and grab their "own" DOIs (which can be a hassle).

We currently have tu_dois(), and that might just be ok, though we should probably ask whether it's ok to use that.

Would also be good to have a dataset which tests all the cases that we're looking at, both for testing and for showcasing.