Updating the documentation from qs to qs2

[LisaNicvert]

This is a very minor detail, but the Getting started with CmdStanR vignette recommends using qs for more efficient saving of fitted objects. But qs is deprecated and is being replaced with qs2, it would be nice to update the documentation if you have the time!

[jgabry]

Thanks for opening the issue! Very recently we actually added support for qs2 directly in the fit$save_object() method in #1125 (to use it you can install cmdstanr from Github). The vignette was also updated, but since we haven't done a release yet, the version of the vignette on the website hasn't updated yet. Recently we also added a "dev" version of the website, which is in sync with the development version of cmdstanr on GitHub, so you can see what the vignette update will look like here: https://mc-stan.org/cmdstanr/dev/articles/cmdstanr.html#saving-fitted-model-objects

[LisaNicvert]

Thanks, for your answer, that looks great. Also exciting to learn that $save_object() will support the qs format in the future!