[Enhancement]: rule derivations online

### Feature/Enhancement Description Currently, we have a lot of rule derivations written down internally in a BIASlab overleaf file. It would be cool to have them available online, perhaps as part of the RxInfer documentation? ### Motivation / Use Case It will enable the community to formally understand message passing faster, compared to having to look up the derivation elsewhere, deriving it themselves or scrutinizing our code. ### Proposed Solution <img width="1005" height="47" alt="Image" src="https://github.com/user-attachments/assets/961ea8c6-c796-4e03-8735-a2b022d0a476" /> a separate tab/location in the RxInfer documentation. We will need some way to write LaTeX equations, and to allow BIASlab members to contribute. ### Alternatives Considered We could also host it on a separate webpage. Possibly easier to implement, but having it in one place is cleaner. ### Example Use Cases The community can level up their knowledge. It makes it easier to become a contributor to RxInfer instead of just a user. ### Priority (from your perspective) Would be helpful ### Related Issues / Discussions _No response_ ### Additional Context _No response_