check() now checks pkgdown

[olivroy]

fix #2449

If the pkgdown config is not okay, then it errors, should it just warn?

If we decide to keep on going, we could use

tryCatch(
  pkgdown::check_pkgdown(),
  error = function(cnd) {
    warning(conditionMessage(cnd), call. = FALSE)
  }
)

Maybe this should be pkgdown_sitrep() after pkgdown 2.1 to avoid tryCatch.

Perhaps this should happen as part of document in the build pane? or check_man()?