Function entity_rel_path 

pub fn entity_rel_path(
    group_dir: &str,
    category_dir: &str,
    name: &str,
) -> String

Relative output path for an entity page under a group’s markdown tree.

Shape: {group_dir}/{category_dir}/{name}.md using forward slashes. Does not include Options::markdown_root.

Full slug parity with protobuf-mdbook is deferred; this strips path separators from name only.