User-friendly version of AR1()

The current ar1() function is flexible but difficult to use. Should create a new version for the stationary ar1(). One of the parameters should be the stationary variance; the other might be either the random-walk variance or the correlation coefficient for successive time steps.

Also, documentation should give guidance on priors.

Start by locating the current ar1() implementation and its existing documentation. Clarify whether the stationary variance should be paired with random-walk variance or successive-step correlation, then define the new user-facing API and the prior guidance. Done means the stationary version and its documentation are implemented with an agreed parameter choice.

Written by the indexing model from the issue text.