doc: Fix headings of the API documentation.

Removes the Auto-generated text from the main heading. Changes the headings
to the consistent hierarchy used elsewhere in the documentation.

Change-Id: I146e4d7ac280b4a3bb81fc3263f826637a8bf08f
Signed-off-by: Rodrigo Caballero <rodrigo.caballero.abraham@intel.com>
Signed-off-by: Anas Nashif <anas.nashif@intel.com>