doc: Move DevStack local.conf to "Topics" section

contributor/ref/ directory contains the module reference and
local.conf was the only exception. "Topics" section looks
more appropriate place to have a guide to configure DevStack
for horizon.

Change-Id: Ibfafe7c608753633e0b78765073afa2924602551

doc/source/contributor/ref/horizon.rst

@@ -1,5 +1,5 @@
 ==================
-The horizon Module
+The Horizon Module
 ==================
 
 .. module:: horizon

doc/source/contributor/ref/index.rst

@@ -1,8 +1,9 @@
-=============
-API Reference
-=============
+================
+Module Reference
+================
 
-In-depth documentation for Horizon framework and its APIs.
+Horizon Framework
+-----------------
 
 .. toctree::
    :maxdepth: 1
@@ -17,10 +18,9 @@ In-depth documentation for Horizon framework and its APIs.
    decorators
    exceptions
    test
-   local_conf
 
-Auth API Reference
-==================
+openstack_auth Module
+---------------------
 
 .. toctree::
    :maxdepth: 2

doc/source/contributor/topics/devstack.rst

@@ -1,11 +1,8 @@
 .. _local-conf:
 
-==========
-local.conf
-==========
-
-Configuring DevStack for Horizon
-================================
+====================
+DevStack for Horizon
+====================
 
 Place the following content into ``devstack/local.conf`` to start the services
 that Horizon supports in DevStack when ``stack.sh`` is run. If you need to use

doc/source/contributor/topics/index.rst

@@ -19,3 +19,4 @@ the following topic guides.
    translation
    ini-based-configuration
    packaging
+   devstack