docs: decruft Documentation/conf.py

Remove the ancient support for the Sphinx "classic" theme; everybody will
have alabaster, so that fallback is no longer needed.

While in the neighborhood: get rid of lots of useless comment lines.  They
describe the state of Sphinx options when we first created that file and
are just clutter now.

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