fixes and improvements for the WebNLG loader
The dataset card is fantastic!

Looks good to me! Did you check that this still passes the slow tests with the existing dummy data? Yes, I ran and passed all the tests specified in [this guide](https://github.com/huggingface/datasets/blob/master/ADD_NEW_DATASET.md#automatically-add-code-metadata), including the slow ones. I just added the `from pathlib import Path` at the top to fix the script I ran the tests locally and they all pass, merging Thank you for the review!
- fixes test sets loading in v3.0
- adds additional fields for v3.0_ru
- adds info to the WebNLG data card