Update git submodules

Update git submodules

* Update docs/submodules/apex from branch 'master'
  - Merge "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

    added index.rst so that docs build succsesfully

    Change-Id: I2d28f03ad4d5c9c14de095dc544257af9d1e51d9
    Signed-off-by: Aric Gardner <agardner@linuxfoundation.org>