guides: Notes to setup DevStack with Nested KVM Virtualization

Add a document with procedure to configure KVM-based nested
virtualization on the physical host and to configure DevStack (in a VM)
to take advantage of it.

Current topics:

   - Configure nested virt on Intel hosts
   - Configure nested virt on AMD hosts
   - Expose virt extensions to DevStack VM
   - Ensure DevStack VM is using KVM

Change-Id: Ibe6fa482cc0d51183438d99680a0e10d0da652cb
diff --git a/doc/source/index.rst b/doc/source/index.rst
index 7188f6e..1aaab6e 100644
--- a/doc/source/index.rst
+++ b/doc/source/index.rst
@@ -66,6 +66,7 @@
    guides/single-machine
    guides/multinode-lab
    guides/neutron
+   guides/devstack-with-nested-kvm
 
 All-In-One Single VM
 --------------------
@@ -94,6 +95,13 @@
 This guide is meant for building lab environments with a dedicated
 control node and multiple compute nodes.
 
+DevStack with KVM-based Nested Virtualization
+---------------------------------------------
+
+Procedure to setup :doc:`DevStack with KVM-based Nested Virtualization
+<guides/devstack-with-nested-kvm>`. With this setup, Nova instances
+will be more performant than with plain QEMU emulation.
+
 DevStack Documentation
 ======================