Add Feature section in doc
Add Feature section in doc

subscription_confirm guide is missing in the doc index.

This patch add a new index section called "Feautre Guide".

More feature guide could be added here, such as "OSprofiler",
"purge queue" and so on.

Change-Id: Ic8d9105e8d69198c507cb0ec581d4e431c96ec40

openstack/zaqar,openstack/zaqar,openstack/zaqar,openstack/zaqar