add accessibility tree to PDF export file (Tagged PDF))

[Erwan-le-Gall]

label: accessibility

When I use structure (headers, lists…) in a Docs document

Then it should be reflected in the corresponding PDF exported file (= Tagged PDF).

reference: https://opensource.adobe.com/dc-acrobat-sdk-docs/pdfstandards/PDF32000_2008.pdf#page=581

This an accessibility requirement.

[Erwan-le-Gall]

ping @cyberbaloo

[Erwan-le-Gall]

cf https://pdfkit.org/docs/accessibility.html