docs: Bump sphinx to 6.2.1

docs: Bump sphinx to 6.2.1

sphinx 5.3.0 fails with Python 3.13.1:

../docs/meson.build:37: WARNING: /home/me/qemu/build/pyvenv/bin/sphinx-build:
Extension error:
Could not import extension sphinx.builders.epub3 (exception: No module named 'imghdr')

../docs/meson.build:39:6: ERROR: Problem encountered: Install a Python 3 version of python-sphinx and the readthedoc theme

Bump sphinx to 6.2.1 and also sphinx_rtd_theme as required for the new
sphinx version.

(jsnow note: this patch bumps the recommended version for Sphinx to
install when it is missing, but allows old versions to be used if they
are present and functional. The version used for building docs on
readthedocs is pinned to the recommended version, 6.2.1.)

Signed-off-by: Akihiko Odaki <akihiko.odaki@daynix.com>
Tested-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: John Snow <jsnow@redhat.com>
Signed-off-by: John Snow <jsnow@redhat.com>
Message-ID: <20250715212848.171879-2-jsnow@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>