Add gallery and script to generate it

[ocefpaf]

Closes #173

This is not ready to be merged yet. We need to decide:

• what "badges" we want: highlight package and keyword
• image placeholder for notebooks that do not have a figure: make it smaller
• Should the gallery be the only way to navigate the notebooks?
• Add the badges in the notebook and parse from it

For the last one we will experiment with a main gallery with all the notebooks in a single place, and another one for each current category we have.

@MathewBiddle we can discuss those things over a call and then paste the results here.