Creates a new, empty outline and makes it the document's, replacing any outline the catalog has.
public PdfOutline CreateOutline()
ISO 32000-2 §7.7.2, Table 29 (Outlines), and §12.3.3, Table 150: an indirect << /Type /Outlines >>. An
outline it replaces stays in the file until a save that removes unreachable objects. A mutation: no read may run concurrently
with it.