Add rvars to FAQ or vignettes

[athowes]

In #281 @athowes writes:

Taken from appendix of #52: how does this rvars stuff work?

obs_prep |>
  as.data.frame() |>
  add_predicted_rvars(fit) |>
  select(.prediction) |>
  head()

Should we be recommending ahead of the draws versions?

[seabbs]

Closed by #547