Inserts a child item titled title at index among the item's children.
public PdfOutlineItem Insert(int index, string title)
The position among the children, from 0 to their number.
The text displayed for the child.
ISO 32000-2 §12.3.3, Table 151. The child is linked through Parent, Prev, Next, First and
Last; the item's Count grows (an item that had no children becomes open), and so does each ancestor's while the
new child is visible.