Adding the WebNLG dataset
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 :) 
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).