Suggest {markdown} for md_convert() only

[k5cents]

The md_convert() function isn't really a user-facing function that fits with the rest of the package (mostly used in testing). So, why not simply suggest the markdown package and then follow the R Packages book advice and just return an error.

# You need the suggested package for this function    
my_fun <- function(a, b) {
  if (!requireNamespace("pkg", quietly = TRUE)) {
    stop("Package \"pkg\" needed for this function to work. Please install it.",
      call. = FALSE)
  }
}