docs/sphinx: Explicitly convert Sphinx paths to str

docs/sphinx: Explicitly convert Sphinx paths to str

Sphinx 7.2+ is switching to using pathlib.Path
instead of str to represent paths. This fixes the
current deprecation warnings and eventual breakage.
This conversion will be a no-op when using older
Sphinx versions.

Signed-off-by: Oliver Faso <erer1243@gmail.com>
Tested-by: Akira Yokosawa <akiyks@gmail.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20230910040811.53046-1-erer1243@gmail.com
Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>