Fix docs build
Fix docs build

Sphinx 3.0.1 complains with:

  horizon/horizon/base.py:docstring of horizon.Panel:33:duplicate object
  description of horizon.Panel.nav, other instance in contributor/ref/horizon,
  use :noindex: for one of them

Fix the two problem.

Change-Id: I98b6212c29dc66cc83a4c337771039796d939561

openstack/horizon,openstack/horizon,ChameleonCloud/horizon,ChameleonCloud/horizon,openstack/horizon,ChameleonCloud/horizon,openstack/horizon,ChameleonCloud/horizon