Add reference to varInferenceStack

[sschmidt23]

Markus is working on a paper that documents the algorithm implemented in varInferenceStack, we should add that reference to the documentation once it is on Arxiv/in a journal

[sschmidt23]

I pinged Markus and he has a description of the variational inference in https://arxiv.org/abs/2211.16516, so we can add a reference to that paper in the code and in any docs pages that mention varInferenceStack.