Colab button
This PR adds a "open in colab" button on the tutorials of our documentation. For each of those tutorials, three notebooks are available: mixed version (with cells PyTorch and TensorFlow), PyTorch-only and TensorFlow only, so hovering on the button makes a dropdown appear with the three different links.

Those notebooks are generated automatically from the docs rst files and the script in the [notebooks repo](https://github.com/huggingface/notebooks/blob/master/utils/convert_doc_to_notebooks.py).