net: docs: add page_pool.rst to index.rst

This file is already in ReST format. Add it to the net
index.rst, in order to make it part of the documentation
body.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>