Gets the page whose Annots array holds the annotation, or null when no page holds it (an annotation reached only
from another one). Not the P entry, which is advisory: see PageObject.
public PdfPage? Page { get; }
ISO 32000-2 §12.5.2: "A given annotation dictionary shall be referenced from the Annots array of only one page." When a file
breaks that rule, this is the first page that listed the annotation, and the others record AnnotationSharedAcrossPages.