Article: add front matter (title, author, toc, ...)
Article: add front matter (title, author, toc, ...)

Group title/subtitle/date/author paragraphs in a GroupedFlowables;
otherwise the relevant metadata is not yet present in document.metadata.
beni55/rinohtype,brechtm/rinohtype,brechtm/rinohtype,beni55/rinohtype,brechtm/rinohtype