docs: Add Patrole overview documentation

This patchset adds a Patrole overview documentation
section which is sorely needed. It combines the previous
usage documentation into it.

Change-Id: Ia7412f2e99f33fbdfd2e60ba54ffdba757d1f886
diff --git a/doc/source/index.rst b/doc/source/index.rst
index 8368262..255fd9a 100644
--- a/doc/source/index.rst
+++ b/doc/source/index.rst
@@ -2,6 +2,14 @@
 Patrole: Tempest Plugin for RBAC Testing
 ========================================
 
+Overview
+========
+
+.. toctree::
+   :maxdepth: 2
+
+   overview
+
 User's Guide
 ============
 
@@ -12,7 +20,6 @@
    :maxdepth: 2
 
    configuration
-   usage
 
 Patrole Installation Guide
 --------------------------