Markdown code blocks with class rmd in roxygen2 documentation end up empty

[salim-b]

roxygen2 allows to write "fenced" Markdown code blocks, optionally with a source code language identifier like md. Such a code block results in .Rd content like

\if{html}{\out{<div class="sourceCode md">}}\preformatted{
CODE BLOCK CONTENT
}\if{html}{\out{</div>}}

But if one specifies the language identifier rmd for a code block, pkgdown silently omits the block's content in the generated HTML.

Try to render the following .R file content to reproduce the issue (and change rmd to md to make pkgdown not omit the Can you see me? block content):

#' Test
#'
#' ``` rmd
#' Can you see me?
#' ```
#'
#' @export
test <- function() {
  "test"
}

I believe this to be a regression somewhere between pkgdown 2.0.6 and 2.0.7, maybe related to changes around #2092.