Add XFUN dataset
The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/datasets/pr_3974). All of your documentation changes will be reflected on that endpoint. Not sure how to generate dummy data.

The downloaded file structure is 

- document file paths
    - (a json file containing all documents info, document images folder)
    - (a json file containing all documents info, document images folder)
    - ... Hey @mariosasko, thanks for the review. I'm not sure how to suggest these changes to the owner @ranpox, and I did spend some time to write the model card and hope to get it on the official repo. Is that possible?
This PR adds XFUN dataset.

Home page and repository: https://github.com/doc-analysis/XFUND

Source code: https://github.com/microsoft/unilm/blob/master/layoutlmft/layoutlmft/data/datasets/xfun.py