Generate bookmarks to each PDF starting point when merging multiple PDF files

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The "merge PDFs" route (POST http://localhost:3000/forms/pdfengines/merge) could include the feature of generating bookmarks in the PDF to each of the combined documents' starting points.

Each of the input PDFs potentially has a title as metadata. This title can be used to add a bookmark in the combined PDF, that points to the first page of that input document in the output document.

See #864

Example

Input: 4 documents titled "Intro" (2 pages), "Chapter 1" (10 pages), "Chapter 2" (12 pages), "Appendix" (3 pages)
Output: A single PDF as usual, but that also has bookmarks with each document's title, and pointing to the corresponding page:

• "Intro" (points to page 1)
• "Chapter 1" (points to page 3)
• "Chapter 2" (points to page 13)
• "Appendix" (points to page 25)

Why?

PDF reader apps often show this as a table of content in their own UI, for easier navigation:

Kindle and similar devices show this table of content as well.