docs: set format for the classic mode

When RTD is not installed or when THEME=classic is used, the
produced docs contain some weird selections. As this theme has
several variables to customize it, set them, in order to produce
a nicer output.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/8889380606681a2b7033f73bed9717250302be2a.1638870323.git.mchehab+huawei@kernel.org


Signed-off-by: Jonathan Corbet <corbet@lwn.net>