Gives the document an empty structure tree and marks it as a tagged PDF.
public PdfStructureTreeRoot CreateStructureTree()
ISO 32000-2 §7.7.2 (Table 29, StructTreeRoot and MarkInfo), §14.7.2 (Table 354), §14.7.5.4, §14.8.1. The root is a
new indirect object with an empty K, an empty parent tree and ParentTreeNextKey 0; the catalog's MarkInfo
gets Marked true. Structure trees need PDF 1.3 and tagged PDF 1.4 (MinimumVersion).
A mutation: no read may run concurrently with it.