Integrate notebook comparing Stan and PyMC

[ricardoV94]

Description

I wrote a notebook comparing how models are defined in Stan and PyMC that other devs found useful and suggested we add to our core docs. If someone has the time to push that, it would be great: https://gist.github.com/ricardoV94/c421ddacb3ba7a19ac46efa253ea466c

[Cocoden7]

Hey, can you attribute it to me? Thanks