Local Documentation Builds

Local Documentation Builds

This adds configuration for performing local documentation builds
with the following simple command:

tox -e docs

This patch is part of our documentation effor to Move away from
submodules
https://docs.opnfv.org/en/latest/how-to-use-docs/local-build-transition.html

Your project is now required to have a docs/index.rst so if you are
missing this file please create it.

Change-Id: I32a91a4194e4afe98f4c964052e99419b22330ba
Signed-off-by: Aric Gardner <agardner@linuxfoundation.org>
(cherry picked from commit 42333625db781f1ae12212ce903df9b5257ff5e4)