Rename rbac_policy_parser to policy_authority

This change is a follow-up to commit

I8ba89ab5e134b15e97ac20a7aacbfd70896e192f

which introduced an abstract class from which (previously)
rbac_policy_parser and requirements authority inherit, providing
rbac_rule_validation with 2 ways of validating RBAC.

For the sake of naming consistency, rbac_policy_parser is renamed
to policy_authority. This naming scheme is better because
"policy parser" is implementation-specific and doesn't convey
what the file (and class name) do from a high-level perspective.

Because this file is only used internally to Patrole, it can be
changed without backward-compatibility concerns.

This commit also includes documentation for the policy authority
module and the rbac_rule_validation module.

Change-Id: Ie09fc2d884f9211244b062fdd5fe018970c2bb2d
diff --git a/doc/source/index.rst b/doc/source/index.rst
index f58ee7f..e2cc0bd 100644
--- a/doc/source/index.rst
+++ b/doc/source/index.rst
@@ -20,9 +20,17 @@
 =================
 
 .. toctree::
+   :maxdepth: 1
+
+   HACKING
+
+Framework
+---------
+
+.. toctree::
    :maxdepth: 2
 
-   contributing
+   rbac_validation
 
 Indices and tables
 ==================