doc: add sphinx numref compatibility fallback

doc: add sphinx numref compatibility fallback

This change adds some simple handling for the :numref: directive
for Sphinx versions prior to 1.3.1. This allows the Guides
documentation to be built with older versions of Sphinx and still
produce reasonable results.

The patch replaces the :numref: reference with a link to the
target (for all Sphinx doc types). It doesn't try to label
figures/tables.

Full numref support with automatic figure/table numbering and
links can be obtained by upgrading to Sphinx 1.3.1 or later.

Signed-off-by: John McNamara <john.mcnamara@intel.com>
Acked-by: Thomas Monjalon <thomas.monjalon@6wind.com>