Function build_source_to_url 

fn build_source_to_url(pages: &[PathBuf]) -> HashMap<(String, String), String>

Build a (parent_dir, file_stem) -> vocs_url map from the emitted MDX pages.

Each page is named <type>.<Name>.mdx; the key mirrors the source .sol file (parent dir + item name), so a README link to path/to/Name.sol matches the page for the item whose name equals the file stem.