docs: mention hardenconfig in kconfig
docs: mention hardenconfig in kconfig

Adds a mention of the Kconfig hardening script hardenconfig to
the Kconfig documentation to promote awareness of its existence.

Signed-off-by: Lauren Murphy <ce8d3478b631de4cc130a07ddd0282d1734fed61@intel.com>

zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,galak/zephyr,galak/zephyr,finikorg/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,finikorg/zephyr,galak/zephyr