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>

doc/about_zephyr.rst

@@ -52,6 +52,6 @@ other small footprint kernels.
   increase performance. An optional error checking infrastructure is provided
   that can assist in debugging during application development.
 
-The Zephyr kernel is suppported on multiple architectures,
+The Zephyr kernel is supported on multiple architectures,
 including ARM Cortex-M, Intel x86, and ARC. The list of supported platforms
 can be found :ref:`here <platform>`.

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`
-

doc/kernel/nanokernel/nanokernel.rst

@@ -16,4 +16,3 @@ applications and nanokernel applications.
    nanokernel_synchronization
    nanokernel_data
    nanokernel_interrupts
-   nanokernel_example