Mention new/extension distributions in docs (e.g. BinomialLogit, Flat, FlatPos, ...)

[JasonPekos]

Just found out today that Turing has a BinomialLogit: https://github.com/TuringLang/Turing.jl/blob/03eba139853d950119821fbcac847b43b0caaf06/src/stdlib/distributions.jl#L45

This is pretty useful, and so should be in the docs, either as part of a tutorial, or in a new section under the documentation tab (e.g. "Convenience Distributions" or something similar).

I can add this when I'm free, so I'm creating this issue as a reminder, and also in case anyone has any feedback on where this should go.

Leaning towards putting this in a doc instead of a tutorial because that'll make it easier to find for new users IMO.