doc: Import configuration reference

Import the following files from the former config-reference [1]:

  api.rst
  cells.rst
  fibre-channel.rst
  hypervisor-basics.rst
  hypervisor-hyper-v.rst
  hypervisor-kvm.rst
  hypervisor-lxc.rst
  hypervisor-qemu.rst
  hypervisor-virtuozzo.rst
  hypervisor-vmware.rst
  hypervisor-xen-api.rst
  hypervisor-xen-libvirt.rst
  hypervisors.rst
  index.rst
  iscsi-offload.rst
  logs.rst
  resize.rst
  samples/api-paste.ini.rst
  samples/index.rst
  samples/policy.yaml.rst
  samples/rootwrap.conf.rst
  schedulers.rst

The below files are skipped as they're already included, in slightly
different forms, in the nova documentation.

  config-options.rst
  nova-conf-samples.rst
  nova-conf.rst
  nova.conf

Part of bp: doc-migration

Change-Id: I145e38149bf20a5e068f8cfe913f90c7ebeaad36

doc/source/index.rst

@@ -167,6 +167,10 @@ Reference Material
 
 * **Configuration**:
 
+  * :doc:`Configuration Guide </admin/configuration/index>`: detailed
+    configuration guides for various parts of you Nova system. Helpful
+    reference for setting up specific hypervisor backends.
+
   * :doc:`Config Reference </configuration/config>`: a complete reference of all
     configuration options available in the nova.conf file.
 
@@ -210,6 +214,7 @@ looking parts of our architecture. These are collected below.
    :hidden:
 
    admin/index
+   admin/configuration/index
    cli/index
    configuration/config
    configuration/sample-config