doc: security: Change link to development model.
doc: security: Change link to development model.

The development model docs are now part of the technical docs.

Signed-off-by: Thomas Ebert Hansen <8aa3e1be091054643bb821254913f17ad2a7f7ba@oticon.com>

nashif/zephyr,GiulianoFranchetto/zephyr,Vudentz/zephyr,Vudentz/zephyr,ldts/zephyr,GiulianoFranchetto/zephyr,galak/zephyr,ldts/zephyr,nashif/zephyr,zephyrproject-rtos/zephyr,ldts/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,GiulianoFranchetto/zephyr,Vudentz/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,ldts/zephyr,finikorg/zephyr,finikorg/zephyr,nashif/zephyr,galak/zephyr,galak/zephyr,finikorg/zephyr,ldts/zephyr,nashif/zephyr,finikorg/zephyr,galak/zephyr,GiulianoFranchetto/zephyr,Vudentz/zephyr,nashif/zephyr,galak/zephyr,GiulianoFranchetto/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr