doc: disable man page section directories for Sphinx 4.0
Sphinx 4.0 creates man page section directories by default.
Our "install-man" target expects a flat directory structure and
is broken when man page section directories are used.
Sphinx's `man_make_section_directory` option was introduced for
this feature and defaults to `True` in Sphinx 4.0 and `False` in
earlier versions.
Disable the `man_make_section_directory` feature so that we can
build uniformly across Sphinx versions.
Closes #1141
Related-to: https://github.com/sphinx-doc/sphinx/issues/7996
Signed-off-by: David Aguilar <davvid@gmail.com>