docs: update Sphinx version to 7.0.1

Sphinx 5 relies on the `imghdr` module from the standard library, but
this module is no longer available in Python 3.11 and up (for the PEP
which justified its removal, [PEP 594][1]).

[1]: https://peps.python.org/pep-0594/#imghdr



Signed-off-by: Chris Kay <chris.kay@arm.com>