Vignette/template demonstrating how to contribute a new plotting function to bayesplot

[jgabry]

Given the increasing number of people developing R packages depending on Stan, it would be nice to have a vignette demonstrating the process of contributing a new plotting function to bayesplot, including the coding (e.g., internally using the available _data functions or making a new one), doc, tests, pull request, etc? Something similar to @tjmahr's blog post but specifically focused on providing a template for new contributors. There are enough packages listing