doc: turbo mode for kconfig options

Building the documentation for all the Kconfig options significantly
adds to the total doc build time.  When making and testing major changes
to the documentation, we provide an option to temporarily stub-out
the auto-generated configuration documentation so the doc build process
runs much faster.

To enable this mode, set the following option when invoking cmake

    -DKCONFIG_TURBO_MODE=1

Signed-off-by: Anas Nashif <anas.nashif@intel.com>