Add: document question answering task guide
This PR adds a task guide on document question answering. 

The example is largely based on @NielsRogge 's [notebook](https://github.com/NielsRogge/Transformers-Tutorials/blob/master/LayoutLMv2/DocVQA/Fine_tuning_LayoutLMv2ForQuestionAnswering_on_DocVQA.ipynb) with some inspuration from [this blog post](https://www.philschmid.de/fine-tuning-layoutlm#3-fine-tune-and-evaluate-layoutlm)

A Colab notebook to test the code examples is [here](https://colab.research.google.com/drive/1vIc-YoGbWdNOBGSb6mpkw9myJ4nr2vcA?usp=sharing).
_The documentation is not available anymore as the PR was closed or merged._<|||||>@MKhalusova is it okay if I reviewed the latest changes on Monday? I am currently on travel. <|||||>> @MKhalusova is it okay if I reviewed the latest changes on Monday? I am currently on travel.

Absolutely!