docs: Add k8s policy primer
docs: Add k8s policy primer

Previously the documentation for writing policies was based on
examples from the traditional compute-networking-storage arena.

This policy primer was purpose-built to introduce people to
writing policy for the Kubernetes admission control use case.

Signed-off-by: Tim Hinrichs <5ee0edb9e2229c0838f1959779f1949031de0123@styra.com>

tsandall/opa,open-policy-agent/opa,tsandall/opa,open-policy-agent/opa,open-policy-agent/opa,tsandall/opa,tsandall/opa,tsandall/opa,open-policy-agent/opa,open-policy-agent/opa,tsandall/opa,open-policy-agent/opa