[WIP] Add VQA docs
This PR adds visual question answering to the pipeline tutorial (under a more general multimodal header) and the fine-tune section of the guides. It would also be nice to create a VQA Tasks video similar to the other fine-tune guides, but this is not a super high priority right now :)

## TODO

- [ ] Create fine-tune guide for VQA.
The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_17907). All of your documentation changes will be reflected on that endpoint.<|||||>This issue has been automatically marked as stale because it has not had recent activity. If you think this still needs to be addressed please comment on this thread.

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