Add PAGE XML export and documentation.
To generate PAGE XML output just add 'page' to the tesseract command.

The output is outputname + '.page.xml' to avoid conflicts with ALTO export.

The output can be customized with the flags:
tessedit_create_page_polygon and tessedit_create_page_wordlevel.

Co-authored-by: Stefan Weil <sw@weilnetz.de>