New posteriors to be added

[MansMeg]

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Below are some updates in the documentation and the multiple new posteriors from @PhilClemson .

Also, new dogs models are being added/improved upon.

[MansMeg]

Hi @bob-carpenter !

Now I'm done with the PR so far. The PR contain many new models contributed by @PhilClemsons group. Most relatively easy one from ARM and winbugs, the new dogs models, an updated contribution information (essentially pointing to the R package) and some minor fixes.

The test goes through, so all new models are checked with rstan that they can be run and generate samples.