Add a vignette that shows use case

[medewitt]

Can be similar to what I did in the intervalcalc package

See:

https://github.com/medewitt/intervalcalc/blob/2b759d5ac483e1358ac3bd02b0148bd805db603b/R/fit_interval_data.R#L20

This one is a bit more complicated because I loop through several Stan fits, but it is something...