Add code folding

[anders-kolstad]

I spent a long time trying to get code folding to work for the fpi wetlands chapter. It works for normal html_documents and also for gitbook but I have not found a way to included it for bs4_books. This is not essential. We can use scrollable code block instead.

[anders-kolstad]

Next time I want to mimick the style used here. Look at the code folding button in the appendix. The repo is not publich yet I think I so cant see the rmd files, but I think they used a boostrap theme here.
@joatop

They also have beatuiful callout boxes that are easy to do with quatro #11

[anders-kolstad]

For this web site we'll just continue to use scrollable code blocks, like this

pre {
  max-height: 300px;
  overflow-y: auto;
}

pre[class] {
  max-height: 200px;
}

, but code folding is easy with quarto, so we can use quarto next time. See my own example here