Use cmdstanr built-in examples

[wlandau]

Prework

• Read and agree to the code of conduct and contributing guidelines.
• If there is already a relevant issue, whether open or closed, comment on the existing thread instead of posting a new issue.
• For any problem you identify, post a minimal reproducible example so the maintainer can troubleshoot. A reproducible example is:
  • Runnable: post enough R code and data so any onlooker can create the error on their own computer.
  • Minimal: reduce runtime wherever possible and remove complicated details that are irrelevant to the issue at hand.
  • Readable: format your code according to the tidyverse style guide.

Description

Replace tar_stan_file_example() and possibly tar_stan_data_example() with cmdstanr examples where possible.

[wlandau]

Changed my mind. We can stick with what we have.