Support prose in examples

[t-kalinowski]

In reprex::reprex() and knitr::spin(), roxygen formatted comments get rendered as prose. It would be great if pkgdown did the same for examples.

Concretely, I'm proposing users could write something like:

#' A title
#'
#' @export
#' @examples
#' #' This gets rendered as prose
#' this_is_rendered_as_monospaced_code <- fn()
#' 
#' #' This also gets rendered as prose
#' this_is_also_rendered_as_monospaced_code <- fn()
fn <- function() { }