Splits the document into one new document per range of pages.
public IReadOnlyList<PdfDocument> Split(params IEnumerable<Range> ranges)
The 0-based page ranges, such as 0..3, 3..7 and 7..; each selects at least one page. They may overlap.
ISO 32000-2 §7.7.3, §12.3, §12.4.2 and §14.7. Each document is made as ExtractPages(Range) makes it, so its outline holds only the items that show its own pages and its structure tree only the elements whose content is on them. Every range is checked before any document is made. This document is only read.