Text export - retain page numbers/ breaks

[vvasuki]

As mentioned in #504 -

Also, given that different pages use identical footnote numbers, it would be very useful for the text export to have page numbers, so that the footnote definition may be easily matched with anchor intra-page.

Another justification - Core texts with commentaries aligned on each page (as in https://ambuda.org/proofing/gdytryn-raamaanujkrtn-sttiikm/79/ ) would be easily separable if page breaks were retained.