This PR closes #544.

Adds reference/model-index.qmd: a single page that prints the source of every Stan model in the package, grouped by the session in which it is first introduced. Reading them in order makes it easier to see how the models build up across the course (the ESPIDAM 2025 participant request).

Implementation notes:

• Programmatic — model code is read from nfidd_stan_path() at render time, so the page always shows the current Stan source.
• A stopifnot(setequal(unlist(session_models), nfidd_stan_models())) guard fails the render (and thus the deploy) if a new .stan file is added without being placed in the index, or vice versa.
• Picks up the auto-sidebar via _quarto.yml contents: "reference/*.qmd". Order 12 puts it just after using-our-stan-models (11).