docs: tweak some Alabaster style parameters

This is just the beginning: tighten up the layout a bit to improve the
information density in the browser.  Also reconfigure the page width in
terms of character units (em) rather than pixels, making it more
display-independent.  To that end, add a custom.css file to
tweak Alabaster CSS settings.

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