Improve Documents with a general introduction to Causal probabilistic programming

[NathanielF]

Taking some inspiration from the Chirho docs here: https://basisresearch.github.io/chirho/tutorial_i.html

We could offer a general introduction to the causal workflow using probabilistic programming languages and show the pymc do-operator too.

[NathanielF]

I'm going to try and pick this up next week