Add Admin doc for an Introduction to Manila

Add doc for an Introduction to Manila dealing with the fundamentals and basic
terminology used with respect to Manila from an Admin perspective.
Also, create a directory for an Admin ref along with a sample index.rst.

Change-Id: I1847f291750227927a747b5ea224a6861310a529

doc/source/adminref/index.rst

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+..
+      Licensed under the Apache License, Version 2.0 (the "License"); you may
+      not use this file except in compliance with the License. You may obtain
+      a copy of the License at
+
+          http://www.apache.org/licenses/LICENSE-2.0
+
+      Unless required by applicable law or agreed to in writing, software
+      distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+      WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+      License for the specific language governing permissions and limitations
+      under the License.
+
+Other Resources
+---------------
+.. toctree::
+   :maxdepth: 3
+
+   launchpad
+   gerrit
+   jenkins
+
+Indices and tables
+------------------
+
+* :ref:`genindex`
+* :ref:`modindex`
+* :ref:`search`
+
+Module reference
+----------------
+.. toctree::
+   :maxdepth: 3
+
+   intro

doc/source/adminref/intro.rst

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+..
+      Licensed under the Apache License, Version 2.0 (the "License"); you may
+      not use this file except in compliance with the License. You may obtain
+      a copy of the License at
+
+          http://www.apache.org/licenses/LICENSE-2.0
+
+      Unless required by applicable law or agreed to in writing, software
+      distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+      WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+      License for the specific language governing permissions and limitations
+      under the License.
+
+Introduction to Manila Shared Filesystem Management Service
+===========================================================
+
+Manila is the File Share service project for OpenStack. To administer the
+OpenStack File Share service, it is helpful to understand a number of concepts
+like share networks, shares, multi-tenancy and back ends that can be configured
+with Manila. When configuring the File Share service, it is required to declare
+at least one back end. Manila can be configured to run in a single-node
+configuration or across multiple nodes. Manila can be configured to provision
+shares from one or more back ends.
+The OpenStack File Share service allows you to offer file-share services to
+users of an OpenStack installation.