doc: updated to reflect changes introduced with Zephyr CMake package

This commit updates the Zephyr documentation with changes introduced
by Zephyr CMake package.

It removes 'zephyr-env.sh/cmd' where no longer needed, and updates
boilerplate inclusion to find_package.

Signed-off-by: Torsten Rasmussen <Torsten.Rasmussen@nordicsemi.no>