Pin docutils for better doc
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The latest release of docutils make the navbar in the documentation weird and the Markdown wrongly interpreted:

![image](https://user-images.githubusercontent.com/35901082/113711773-5be55280-96b3-11eb-9b3b-9794f17709aa.png)

We had the same problem in Transformers and solved it by pinning docutils (a dep of sphinx).

You can see the version after the change [here](https://32769-250213286-gh.circle-artifacts.com/0/docs/_build/html/index.html).

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