docs: set man_make_section_directory to false
docs: set man_make_section_directory to false

And fix inconsistent manpage build paths between recent sphinx
releases. False is the currently expected value, as setup.py
will check for a built manpage in `docs/_build/man/streamlink.1`
for inclusion in the release tarballs.

Even though the doc's HTML theme "furo" does currently limit the
used sphinx version to 3.x, this won't stop users building the
manpage using sphinx 4.x, eg. via a global system-package.

melmorabity/streamlink,bastimeyer/streamlink,chhe/streamlink,streamlink/streamlink,streamlink/streamlink,chhe/streamlink,gravyboat/streamlink,gravyboat/streamlink,bastimeyer/streamlink,melmorabity/streamlink