Inserts a top-level item titled title at index.
public PdfOutlineItem Insert(int index, string title)
The position among the top-level items, from 0 to their number.
The text displayed for the item.
ISO 32000-2 §12.3.3, Tables 150 and 151. The item is linked to its neighbours through Prev and Next and to the
outline through Parent, First and Last, and the outline's Count grows by one. The positions are
those of the outline's links now, not of an earlier Items walk. A mutation: no read may run concurrently with it.