Gets the root of the document's structure tree, or null when the document has none.
public PdfStructureTreeRoot? StructureTree { get; }
ISO 32000-2 §7.7.2, Table 29 (StructTreeRoot), §14.7.2. Read when first asked for; an untagged document has no structure
tree and no diagnostic. A StructTreeRoot that is not a dictionary is ignored with a StructTreeRootInvalid diagnostic.
The views share indexes built on first use (see PdfStructureTreeRoot); they are rebuilt when anything they were
built from changes, and when the catalog's StructTreeRoot entry is replaced. CreateStructureTree() adds one.