Function docname_join 

pub fn docname_join(base_docname: &str, docname: &str) -> String

Sphinx docname_join (util/__init__.py): posixpath.normpath(posixpath.join('/' + basedocname, '..', docname))[1:] — a leading / makes the target source-root-relative, anything else is relative to the referencing document’s directory, and ./.. segments are normalized.