Additional step to be documented for removing policy assignment using ALZ terraform accelerator

[chianw]

Currently the steps to remove a policy assignment when using ALZ terraform accelerator are described at https://azure.github.io/Azure-Landing-Zones/accelerator/startermodules/terraform-platform-landing-zone/options/policy-assignment/

However, this is missing one last additional step of assigning the override archetype to a management group as described in https://azure.github.io/Azure-Landing-Zones/terraform/howtos/modifyingPolicyAssets/#step-three---assign-the-override-archetype-to-a-management-group

Without the last step the policy assignment removal will not work correctly.