rewrite HMM section of User's Guide to use new functions

[bob-carpenter]

Summary:

The section on HMMs in the user's guide should be rewritten to use the new(ish) built-ins

Description:

Replace this user's guide section: https://mc-stan.org/docs/2_29/stan-users-guide/hmms.html

With calls to this: https://mc-stan.org/docs/2_29/functions-reference/hidden-markov-models.html

There's already a case study from which to draw examples: https://mc-stan.org/users/documentation/case-studies/hmm-example.html

Current Version:

v2.18.0

[jsocolar]

I suggest refraining from deletion of the current material unless/until the hmm* functions can handle non-stationary transition probabilities.