Move the kernel documentation up and make it a main chapter. Right now
it is hidden very low in the structure under references.
Signed-off-by: Anas Nashif <anas.nashif@intel.com>
Most of the content in the default index is HTML only, so create an
index just for PDF and keep it clean.
Signed-off-by: Anas Nashif <anas.nashif@intel.com>