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