doc: fix build with Sphinx 4

doc: fix build with Sphinx 4

Sphinx 4.0 became stricter with permalink configuration:
"
html_add_permalinks has been deprecated since v3.5.0.
Please use html_permalinks and html_permalinks_icon instead.
"

The new variable is used while keeping compatibility
with older Sphinx versions.

Cc: stable@dpdk.org
Signed-off-by: Thomas Monjalon <thomas@monjalon.net>
Acked-by: Bruce Richardson <bruce.richardson@intel.com>
Reviewed-by: David Marchand <david.marchand@redhat.com>