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? Since the author is not responding, maybe we can go ahead with this PR ? Go for it!

On Tue, Apr 12, 2022 at 10:24 AM Quentin Lhoest ***@***.***>
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> Since the author is not responding, maybe we can go ahead with this PR ?
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 @qqaatw Do you plan to finish this PR? I can give you some pointers and help you with the code if needed. @mariosasko Yes, I'll apply all of the suggestions when I have some time.
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