docs: driver-api: properly format ToC headings

"class:: toc-title" was a workaround for older Sphinx versions that are
no longer supported.

The canonical way to add a heading to the ToC is to use :caption:.
Do that.

Cc: Mauro Carvalho Chehab <mchehab@kernel.org>
Cc: Richard Cochran <richardcochran@gmail.com>
Cc: SeongJae Park <sj@kernel.org>
Cc: Hans de Goede <hdegoede@redhat.com>
Cc: Armin Wolf <W_Armin@gmx.de>
Cc: Iwona Winiarska <iwona.winiarska@intel.com>
Cc: Ricardo Cañuelo <ricardo.canuelo@collabora.com>
Cc: Tomas Winkler <tomas.winkler@intel.com>
Cc: linux-media@vger.kernel.org
Cc: Yanteng Si <siyanteng@loongson.cn>
Cc: Alex Shi <alexs@kernel.org>
Cc: Wu XiangCheng <bobwxc@email.cn>
Signed-off-by: Vegard Nossum <vegard.nossum@oracle.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20231027081830.195056-7-vegard.nossum@oracle.com