VQA task guide
This PR adds a new Visual Question Answering task guide to the transformers docs: 
fine-tuning ViLT, based on @NielsRogge 's [notebook](https://github.com/NielsRogge/Transformers-Tutorials/blob/master/ViLT/Fine_tuning_ViLT_for_VQA.ipynb)

 
The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_25244). All of your documentation changes will be reflected on that endpoint.