Document a CmdStan-focused way to pre-compile Stan models in R packages

[wlandau]

Submission Checklist

• [n/a] Run unit tests (this PR only modifies a vignette)
• Declare copyright holder and agree to license (see below)

Summary

As proposed by @jgabry at #738 (comment), this PR documents one way to pre-compile Stan models in a CmdStanR-powered R package. instantiate is still in its early days, and I am not sure how to deal with the compiler issues we will inevitably face going forward (c.f. #787, wlandau/instantiate#1). But there are good reasons to pre-compile Stan models in an R package, and this is a start using the modern CmdStan-based tooling.

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):

Eli Lilly and Company

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/)

[jgabry]

Looks good, thanks!