add new section in HMM tutorial

[JasonPekos]

This adds a section to the HMMs tutorial that has a much faster and more concise version of the model (four lines!) using HiddenMarkovModels.jl.

There's a subsection after that which shows how to recover the state estimates with the Viterbi algorithm.

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This closes:

• https://github.com/TuringLang/docs/issues/330