update docs for consistency

adjust the index file to be more readable for the field guide

adjust titles to make more sense when displayed in top level index

change theme to 'nature', which is closer to the openstack default
style. Will eventually write our own style here.

Change-Id: I4f5fed65f9e26a4e4d333b1be4ed02c9ec246b9a
diff --git a/doc/source/index.rst b/doc/source/index.rst
index e37e250..1ca7344 100644
--- a/doc/source/index.rst
+++ b/doc/source/index.rst
@@ -3,6 +3,7 @@
    You can adapt this file completely to your liking, but it should at least
    contain the root `toctree` directive.
 
+===================================
 Tempest Testing Project
 ===================================
 
@@ -14,10 +15,15 @@
    overview
 
 
-Field Guide:
+-------------------------------
+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.
 
 .. toctree::
-   :maxdepth: 2
+   :maxdepth: 1
 
    field_guide/index
    field_guide/api
@@ -28,6 +34,7 @@
    field_guide/whitebox
 
 
+==================
 Indices and tables
 ==================