Document the draws_of trick to emulate rstan::extract()

[jgabry]

#183

Closes #632 and closes #183

Submission Checklist

• Run unit tests
• Declare copyright holder and agree to license (see below)

Summary

Better documents how to obtain structured output similar to rstan::extract() using a combination of cmdstanr and posterior.

Copyright and Licensing

Please list the copyright holder for the work you are submitting
(this will be you or your assignee, such as a university or company):
Columbia University

By submitting this pull request, the copyright holder is agreeing to
license the submitted work under the following licenses:

• Code: BSD 3-clause (https://opensource.org/licenses/BSD-3-Clause)
• Documentation: CC-BY 4.0 (https://creativecommons.org/licenses/by/4.0/)

[andrjohns]

Nice