Builds the document's layout tree: its logical content over every page, from the structure tree when the document is tagged.
public LayoutTree ExtractLayout()
ISO 32000-2 §14.8.4, §14.8.2.5 and ISO/TS 32005; see LayoutTree and LayoutNodeKind. Every page is extracted once as ExtractText() does (artifacts and hidden content left out), and each node is placed on the pages its content is on. Pages that are not tagged contribute no nodes, only their text.