Allocates the next MCID of page's content and makes that marked-content sequence the element's last content item.
public PdfMarkedContentReference AddMarkedContent(PdfPage page)
ISO 32000-2 §14.7.5.2 (Table 357), §14.7.5.4 (Table 359). The page gets a StructParents key when it has none
(ParentTreeNextKey advances), and its parent-tree array lists this element at the MCID: the MCID is the first one that
array and the tree's K entries do not use. MCIDs already in the content but in neither are not seen; pass them to
AddMarkedContent(PdfPage, Int32). The item is written as an integer when the element's Pg is the page (an
element without Pg gets this page), else as an MCR dictionary with its own Pg.