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>
1 file changed