Docs: Add RBAC overview documentation

This patchset adds an RBAC overview documentation section dedicated
to:

* Defining what RBAC is
* Policy in code and validation info related to it
* Custom policies and validation info related to it
* Multiple policies and validation info related to it
* Error codes
* Glossary

This way, users can learn about what RBAC is from a high level
and how Patrole uses validation to validate that it is working
correctly.

Change-Id: Ib411e4d06210135f7bd1cb90d5b6d59da2e5d076
diff --git a/doc/source/index.rst b/doc/source/index.rst
index 255fd9a..001ca32 100644
--- a/doc/source/index.rst
+++ b/doc/source/index.rst
@@ -10,6 +10,14 @@
 
    overview
 
+RBAC Overview
+-------------
+
+.. toctree::
+   :maxdepth: 2
+
+   rbac-overview
+
 User's Guide
 ============