Add example of gradient CCDF plots to slabinterval vignette

[mjskay]

It's a relatively straightforward re-use of ccdfinterval:

df %>%
  ggplot(aes(x = group, y = value, fill = subgroup)) +
  stat_ccdfinterval(aes(thickness = 1, slab_alpha = stat(f)), position = "dodge") +
  ggtitle("stat_ccdfinterval(position = 'dodge')") 

h/t to @Raoul-Kima for suggesting it