doc: Fix headings in master index file.

Search engines recommend having only one heading on level 1.
Modified the headings in the documentation index file to
follow the recommendation and help ensure that search engines
place Zephyr higher on their results lists.

Change-Id: I5590138f88721d1f2cdcbb49ad2aaad1d2dcd466
Signed-off-by: Rodrigo Caballero <rodrigo.caballero.abraham@intel.com>
diff --git a/doc/index.rst b/doc/index.rst
index 41818cc..6a21e8d 100644
--- a/doc/index.rst
+++ b/doc/index.rst
@@ -1,8 +1,11 @@
 ..
     Zephyr Project documentation master file
 
+Zephyr Project Documentation
+############################
+
 Getting Started
-###############
+***************
 
 .. toctree::
    :maxdepth: 1
@@ -11,9 +14,8 @@
    getting_started/getting_started.rst
    kernel/kernel.rst
 
-
 Developer Guides
-################
+****************
 
 .. toctree::
    :maxdepth: 1
@@ -23,27 +25,24 @@
    api/api.rst
    collaboration/collaboration
 
-
 Reference Guides
-################
+****************
 
 .. toctree::
    :maxdepth: 1
 
    reference/kconfig/index.rst
 
-
 Supported Boards
-################
+****************
 
 .. toctree::
    :maxdepth: 2
 
    board/board.rst
 
-
-Indices and tables
-##################
+Indices and Tables
+******************
 
 * :ref:`genindex`