Add necessary new front files
Author: mishig25
Pull request: yes

# What does this PR do?

1. Add new file toctree.yml, which serves the same purose as index.rst for Sphinx (i.e. the entire outline of the docs)
2. Add new file versions.yml, which specifices which versions are available (and whether it should be redirected to sphinx website)
3. Changes from https://github.com/huggingface/transformers/pull/14476

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Fixes # (issue)


## Before submitting
- [ ] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
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      Pull Request section?
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Models:

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