doc: Restore main index and cleanup

Also move about_zephyr back to main page and remove duplicated
examples for nano kernel.

Change-Id: Icbc899ce5688d853a59dfc5c8d50f34048ed212b
Signed-off-by: Anas Nashif <anas.nashif@intel.com>
diff --git a/doc/index.rst b/doc/index.rst
index 05d0787..2cee9f0 100644
--- a/doc/index.rst
+++ b/doc/index.rst
@@ -6,23 +6,39 @@
 
 .. include:: disclaimers.rst
 
-The Zephyr Kernel Documentation
-###############################
+Getting Started
+###############
 
 .. toctree::
-   :maxdepth: 2
+   :maxdepth: 1
 
-   quick_start/quick_start
-   kernel/kernel
-   application/application
-   platform/platform
+   about_zephyr.rst
+   kernel/kernel.rst
+   quick_start/quick_start.rst
+
+Developer Guides
+################
+
+.. toctree::
+   :maxdepth: 1
+
+   application/application.rst
+   platform/platform.rst
    collaboration/collaboration
-   reference/reference
 
-Indexes and Tables
-******************
+Reference Guides
+################
+
+.. toctree::
+   :maxdepth: 1
+
+   reference/api.rst
+   reference/configuration.rst
+   reference/kbuild.rst
+
+Indices and tables
+##################
 
 * :ref:`genindex`
 
 * :ref:`search`
-