configguide: make sections relevant by adding sub-level toctree

configguide: make sections relevant by adding sub-level toctree

This patch wrap each import document with 'toctree' directive so that
we can make sure the sections in imported docs won't take any upper
level section.

This patch also fixes the followings:

* path to copper/featureconfig.rst

* path to promise/configguide/images/

Change-Id: I52d4e9cd637af5707af418ef287d57d8c301b85e
Signed-off-by: Ryota MIBU <r-mibu@cq.jp.nec.com>
(cherry picked from commit b7519539bf10181d248b69486c490df46693b209)