Adding the WebNLG dataset
This PR adds data from the WebNLG challenge, with one configuration per release and challenge iteration.

More information can be found [here](https://webnlg-challenge.loria.fr/)

Unfortunately, the data itself comes from a pretty large number of small XML files, so the dummy data ends up being quite large (8.4 MB even keeping only one example per file).
This is task is part of the GEM suite so will actually need a more complete dataset card. I'm taking a break for now though and will get back to it before merging :) 