docs-rst: better adjust margins and font size

As we have big tables, reduce the left/right margins and decrease
the point size to 8pt. Visually, it is still good enough, and
now less tables are too big to be displayed.

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>