unpaper: add manpage

[milahu]

$ unpaper --help
unpaper 7.0.0
License GPLv2: GNU GPL version 2.
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.

Usage: unpaper [options] <input-file(s)> <output-file(s)>

Filenames may contain a formatting placeholder starting with '%%' to insert a
page counter for multi-page processing. E.g.: 'scan%%03d.pbm' to process files
scan001.pbm, scan002.pbm, scan003.pbm etc.

See 'man unpaper' for options details
Report bugs at https://github.com/unpaper/unpaper/issues


$ man unpaper
No manual entry for unpaper

manpage source
https://github.com/unpaper/unpaper/raw/main/doc/unpaper.1.rst