Feature: Process Inter-page hyperlinks to PDF destinations when aggregating pages.

[nbanyan]

Currently, when using aggregation hyperlinks between pages link back to the served website (or are render as disabled links if using mkdocs build).

Likely this would involve modifying the aggregator to:

• Catalog and add destinations to for heading
• Assess every link and replace with a link annotation if the link maps to a destination (and convert to a disabled link if it doesn't)

Doing this will make the aggregated PDF a more unified product rather than a collection of related but independent artifacts. It will also make generating a aggregated Table of Contents practical (it might even be possible to add page numbers to the Table of Contents if the aggregator generates it alongside the PDFWriter).