Switch to using lfdocs-conf to pull in theme

Switch to using lfdocs-conf to pull in theme

lfdocs-conf is a new project that allows us to centrally manage
project theme configuration without having to duplicate it
everywhere or have the "docs" project become a large submodule
project.

We will push to ReadTheDocs separately and then create a new path
for this doc under docs.opendaylight.org/projects/integration-packaging

For further details refer to:
http://docs.releng.linuxfoundation.org/en/latest/project-documentation.html

Change-Id: I976202be6becc2ff3355bd75505bbfd8a049a888
Signed-off-by: Thanh Ha <thanh.ha@linuxfoundation.org>