docs: style toctree captions as headings

The rtd theme already styles toctree captions as headings, but the
alabaster theme doesn't. Add this in.

Cc: Mauro Carvalho Chehab <mchehab@kernel.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Vegard Nossum <vegard.nossum@oracle.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20231027081830.195056-2-vegard.nossum@oracle.com