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

Documentation/sphinx-static/custom.css

@@ -8,6 +8,9 @@ div.body h1 { font-size: 180%; }
 div.body h2 { font-size: 150%; }
 div.body h3 { font-size: 130%; }
 
+/* toctree captions are styled like h2 */
+div.toctree-wrapper p.caption[role=heading] { font-size: 150%; }
+
 /* Tighten up the layout slightly */
 div.body { padding: 0 15px 0 10px; }
 div.sphinxsidebarwrapper { padding: 1em 0.4em; }