doc: fix man page build for Sphinx 4.x

Sphinx 4.0 changed the default value of man_make_section_directory
from False to True. We create the section directories and move the
files manually, so fix the immediate man build failure by disabling
the feature.

The Sphinx documentation on this [1] is confusing, and has the change
backwards. Git history says the default changed from False to True.

[1] https://www.sphinx-doc.org/en/master/usage/configuration.html#confval-man_make_section_directory