feat(docs): navigate with left/right arrow keys
Author: jeffydc
Pull request: yes

# What does this PR do?

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Enables docs navigation with left/right arrow keys. It can be useful for the ones who navigate with keyboard a lot.
More info : https://github.com/sphinx-doc/sphinx/pull/2064

You can try here : https://174105-155220641-gh.circle-artifacts.com/0/docs/_build/html/index.html

## Before submitting
- [x] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
- [x] Did you read the [contributor guideline](https://github.com/huggingface/transformers/blob/master/CONTRIBUTING.md#start-contributing-pull-requests),
      Pull Request section?
- [ ] Was this discussed/approved via a Github issue or the [forum](https://discuss.huggingface.co/)? Please add a link
      to it if that's the case.
- [ ] Did you make sure to update the documentation with your changes? Here are the
      [documentation guidelines](https://github.com/huggingface/transformers/tree/master/docs), and
      [here are tips on formatting docstrings](https://github.com/huggingface/transformers/tree/master/docs#writing-source-documentation).
- [ ] Did you write any new necessary tests?


## Who can review?

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@sgugger 

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