docs: index.rst: don't use genindex for pdf output

The genindex logic is meant to be used only for html output, as
pdf build has its own way to generate indexes.

Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Acked-by: Vinod Koul <vkoul@kernel.org> # dmaengine and soundwire
Signed-off-by: Jonathan Corbet <corbet@lwn.net>