Function page_relative 

fn page_relative(prefix: &str, slug: &str) -> String

One site URL’s path, as a Markdown link target relative to a page at prefix.

concepts/storage/ becomes ../concepts/storage.md: mkdocs rewrites the .md to the built directory URL, and keeping it is what puts the link under validation.links.not_found. An #anchor survives, so validation.links.anchors gets the other half.