Tidy up document index page

This commit tidies up the tempest document index page. The tempest docs
are increasing day by day. It's really good however I feel it's a little
messy these days. So let's tidy up :)

Change-Id: I4d34934b8d1a157bf38a8f2cafe5de1ba0e6098c

doc/source/index.rst

@@ -2,19 +2,14 @@
 Tempest Testing Project
 =======================
 
-Contents:
+--------
+Overview
+--------
 
 .. toctree::
    :maxdepth: 2
 
    overview
-   HACKING
-   REVIEWING
-   plugin
-   plugin-registry
-   library
-   microversion_testing
-   test-removal
 
 ------------
 Field Guides
@@ -32,6 +27,10 @@ where your test contributions should go.
    field_guide/stress
    field_guide/unit_tests
 
+=========
+For users
+=========
+
 ---------------------------
 Tempest Configuration Guide
 ---------------------------
@@ -56,6 +55,41 @@ Command Documentation
    workspace
    run
 
+==============
+For developers
+==============
+
+-----------
+Development
+-----------
+
+.. toctree::
+   :maxdepth: 2
+
+   HACKING
+   REVIEWING
+   microversion_testing
+   test-removal
+
+-------
+Plugins
+-------
+
+.. toctree::
+   :maxdepth: 2
+
+   plugin
+   plugin-registry
+
+-------
+Library
+-------
+
+.. toctree::
+   :maxdepth: 2
+
+   library
+
 ==================
 Indices and tables
 ==================