[Epic]: message passing rule documentation

[ThijsJenneskens]

Currently, we have message passing rule derivations for (most?) of the nodes available internally in an Overleaf file. Some things can be improved, such as making it publicly available on a website like the book of statistical proofs, and restructuring it. The documentation for specific nodes can be marked features, which can be further divided into tasks (derivation, implementing in RxInfer and adding tests)

[bvdmitri]

I really like this idea!! I also think that Documenter.jl should already support LaTeX so we should be able to put the equations directly in our documentation markdown files. A couple of questions good to consider:

• where is the best place to put it? Naturally ReactiveMP is closer to the rules, but RxInfer repo (and documentation) has more observability. We may also consider a separate website.
• We have the idea of moving all the nodes outside of ReactiveMP repo and make a separate repository for custom node e.g. HGFNode such that users write smth like using AutoregressiveNode, HGFNode. This idea never materialized though but I still have it in the background of my mind

Nevertheless, we need to start somewhere, and this somewhere can be ReactiveMP repository too