docs: remove blank pages from PDF documentation

The PDF documentation download has a lot of blank pages. This is
because Sphinx starts each section on an odd numbered page for
duplex printing. This patch fixes this to allow sections to start
on any page.

Change-Id: I1ba8a4707c39b54205f2a3c9b47c1c21a3fedcb9
Signed-off-by: Elizabeth Ho <elizabeth.ho@arm.com>