Update document theme and section headers

This commit updates the document theme. It is clear, simple, up-to-date
and beautiful. And this commit also updates section headers to fix the
document index.

Change-Id: Ia1e8e16e5f5ddb06e4c4ccb4979b0750b88a4db4
diff --git a/doc/source/index.rst b/doc/source/index.rst
index 1264ecc..c4affd2 100644
--- a/doc/source/index.rst
+++ b/doc/source/index.rst
@@ -2,18 +2,16 @@
 Tempest Testing Project
 =======================
 
---------
 Overview
---------
+========
 
 .. toctree::
    :maxdepth: 2
 
    overview
 
-------------
 Field Guides
-------------
+============
 Tempest contains tests of many different types, the field guides
 attempt to explain these in a way that makes it easy to understand
 where your test contributions should go.
@@ -26,11 +24,9 @@
    field_guide/scenario
    field_guide/unit_tests
 
-=========
 For users
 =========
 
----------------------------
 Tempest Configuration Guide
 ---------------------------
 
@@ -40,7 +36,6 @@
    configuration
    sampleconf
 
----------------------
 Command Documentation
 ---------------------
 
@@ -53,11 +48,9 @@
    workspace
    run
 
-==============
 For developers
 ==============
 
------------
 Development
 -----------
 
@@ -70,7 +63,6 @@
    test_removal
    write_tests
 
--------
 Plugins
 -------
 
@@ -80,7 +72,6 @@
    plugin
    plugin-registry
 
--------
 Library
 -------
 
@@ -89,7 +80,6 @@
 
    library
 
-==================
 Indices and tables
 ==================