Colab integration

Ideally, all the documentation pages based off of Jupyter notebooks should have a button to launch them in Colab.

For example, fastai's documentation has such a button:

First step would be a tutorial on how to set up Colab with Julia and install the dependencies.

See [this Discourse post](https://discourse.julialang.org/t/julia-on-google-colab-free-gpu-accelerated-shareable-notebooks/15319) that shows how to get Julia running on Colab.