docs: Change <h4> style to use smaller font size than <h3>

The docs currently have <h3> and <h4> set to the same font size which
makes headings hard to distinguish. <h1> to <h3> already have entries in
sphinx-static/custom.css to shrink their size a bit from the alabaster
theme.

Add <h4> to custom.css and set it to be smaller than <h3>.
Signed-off-by: Donald Hunter <donald.hunter@gmail.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20231212213137.98453-1-donald.hunter@gmail.com

Documentation/sphinx-static/custom.css

@@ -7,6 +7,7 @@
 div.body h1 { font-size: 180%; }
 div.body h2 { font-size: 150%; }
 div.body h3 { font-size: 130%; }
+div.body h4 { font-size: 110%; }
 
 /* toctree captions are styled like h2 */
 div.toctree-wrapper p.caption[role=heading] { font-size: 150%; }