Function doc_dir_of 

fn doc_dir_of(scope: &str) -> String

The doc directory a source scope’s pages sit in.

crates/atune/src/gp → atune/gp, crates/atune/src → atune. The crate’s own name is the first path segment of every page in its tree, which is what check_crates_cover_rust reads to apply §12’s rule.