new User's Guide chapters/sections

I need a place to keep track of potential additions to the User's Guide. We can just tick them off or delete as they're finished.

• user-defined constrained parameter chapter
• Markov random fields chapter including phi^4 example, and maybe conditional random fields
• ICAR and BYM2 models and discussion of penalized complexity
• structural equation models
• scientific ODE examples like Lotka-Volterra and SIR
• rating, ranking, and diagnostic testing (sensitivity/specificity, IRT, Bradley-Terry, Plackett-Luce)
• R2D2 and horseshoe priors
• imputing contingency matrices
• bandit problems and Thompson sampling
• vector autoregressive models in the time-series chapter
• simple neural network examples (e.g., MNIST digits MLP plus maybe convolution)
• n-gram language models and naive Bayes classifiers
• ANOVA

This issue is a tracking list for potential User's Guide additions; it names no file, test, or entry point. Start by locating the User's Guide source in the documentation repository, then select and scope one unchecked topic with the maintainers. Done means that topic is documented and its checklist item can be marked complete.