add signature fields

This can be done using pyhanko library (in conda-forge) as follows:

pyhanko sign addfields --field 1/20,100,300,400/Editor input.pdf output-with-field.pdf

This could be done automatically in the Makefile perhaps. It would require knowing how many fields to add, and getting the bounding box for each correct - in the template, can we rely on them being in the same place I guess?

I tested this and the output document can be signed with a cerficate using Adobe Acrobat or LibreOffice