doc: fix broken Sphinx by updating to Sphinx 5.0.2

Sphinx 4.x is way past EOL and due to it not pinning its dependencies,
it's effectively broken. See
https://github.com/sphinx-doc/sphinx/issues/11890

 The recommended fix,
although not ideal in the context of an LTS branch, is to update to
Sphinx 5.0.2, which should have minimal impact of how the rendered
documentation looks.

Signed-off-by: Benjamin Cabé <benjamin@zephyrproject.org>