Use Sphinx-Gallery

[ddundo]

Rather than having a list of demos, I thought it would be nicer to use Sphinx-Gallery for a better and more appealing look

[ddundo]

What a great extension! It can do so much. Apart from the above, it can do:

• what we want to do in Converting demos from py to ipynb #34
• in the docs, alongside each method, it can automatically link to demos that use that method (for example, scroll all the way down here https://matplotlib.org/stable/api/_as_gen/matplotlib.pyplot.colorbar.html)
• ...