docs: improve the HTML formatting of kerneldoc comments

Make a few changes to cause functions documented by kerneldoc to stand out
better in the rendered documentation.  Specifically, change kernel-doc to
put the description section into a ".. container::" section, then add a bit
of CSS to indent that section relative to the function prototype (or struct
or enum definition).  Tweak a few other CSS parameters while in the
neighborhood to improve the formatting.

Acked-by: Mauro Carvalho Chehab <mchehab@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>