Function document_title 

pub fn document_title(doctree: &Doctree) -> Node

env.titles[docname]: a fresh title node holding the first section title’s contents, filtered exactly like a toc entry’s.

Port of TitleCollector.process_doc (collectors/title.py:27). Its longtitles differ only when the document carries a title attribute (set by the title directive / <meta>), which nothing produces yet — so the caller stores this same node under both keys.