Add search capability to the docs

This follows the nova docs and adds an in-tree neutron-specific
docs search and an openstack-wide search.

The former is necessary for finding things that aren't obviously
linked from top level indexes, like documentation about the
metadata service proxy (which is what I was originally trying
to find without the help of a search box).

Change-Id: I86dde1936d0c7d445f8a6db182f5f41dfb179752

doc/source/index.rst

@@ -85,3 +85,11 @@ API Extensions
 --------------
 
 Go to https://developer.openstack.org/api-ref/network/ for information about the OpenStack Network API and its extensions.
+
+Search
+------
+
+* :ref:`Neutron document search <search>`: Search the contents of this
+  document.
+* `OpenStack wide search <https://docs.openstack.org>`_: Search the wider
+  set of OpenStack documentation, including forums.