Add (auto-generated) Colabs to all HOWTOs

[Marc van Zee]

Currently our HOWTOs often have nice side-by-side views, but they also hide some code.
It would be useful for users to be able to just copy/paste all code from a Colab and run it separately.

The idea is to then include a link to the Colab at the top of the HOWTO.

Ideally, we would generate either the Colabs from the HOWTOs or the HOWTOs from the Colab, so that we don't have to maintain two copies (as mentioned by @andsteing in #1325 as well).

@cgarciae had some ideas on how to do this as well.

[Andreas Steiner]

This would also solve #1325

[Cristian Garcia]

👀

[Marc van Zee]

Closing this in favor of #1999.