this website is rendered from simple markdown files using the parsedown.php script. You can find this markdown parser on github and some documentation here: https://parsedown.org/
normal URL Links will be parsed by parsedown. But we need a mechanism to link other markdown files in such a way, that they will be processed by index.php and subsequently by parsedown, too. On top, the current path must be added.
such links are - for example - generated by the markdown editor Zettlr: https://www.zettlr.com/
just an image: