Adds a structure element of type as the last top-level element.
public PdfStructureElement AddElement(PdfStructureType type)
The element's structure type and namespace.
P is the root.ISO 32000-2 §14.7.2, Table 355; §14.7.4, §14.8.6.2. An element in the default namespace (Pdf17)
gets no NS entry; any other gets an NS reference to a namespace dictionary listed in Namespaces, which is
added (PDF 2.0) when the document has none of that name. A mutation: no read may run concurrently with it.