[doc] [debian] Add missing stdeb dependencies

Created by: pacien

• python3-pyocr is already available through the Debian repository, no need to use pypi-install.
• python3-{enchant,levenshtein,natsort,pycountry,termcolor} are required to run pypi-install paperwork-backend.
• pypi-install simplebayes was missing.
• pypi-install pyinsane2 may require running pypi-install --release 1.3.7 nose separately.

Those dependencies are required during installation, prior being able to run paperwork-shell chkdeps paperwork{,_backend}.