I don't know how consistent it is with "pandoc-flavored" markdown... but `:::boo` for example is provided by markdown-it plugins via NPM (which I've been using in the 11ty site generator but could be used in many different nodejs platforms other than 11ty) https://github.com/markdown-it/markdown-it-container provides that syntax and there numerous other markdown-it plugins providing other syntax elements https://www.npmjs.com/package/markdown-it.
Is anyone aware of a CMS (like MkDocs, Pico, etc.) that uses pandoc-flavored Markdown? I'm using pandoc to make pdfs, but need some things other platforms don't offer, like fenced html, or is it fenced markdown ( :::boo creating a <div class="boo"> tag).
I'm trying to avoid rolling my own, but I will if I have to I guess.