Update K04-policy-enforcement.md

[Ali-Yazdani]

Adding a quick description of tools (OPA Gatekeeper, Kyverno, and Kubewarden).

[elgalu]

Another idea: to state the differences per Language, Scope and Architecture.

e.g. Kyverno vs Gatekeeper core differences:

• Language: Kyverno policies are written in YAML, while Gatekeeper policies are written in Rego (a declarative language).

• Scope: Kyverno is a Kubernetes-native policy engine that focuses on validating and mutating resources at the time of admission. In contrast, Gatekeeper is a general-purpose policy engine that can be used to validate any kind of resource, not just Kubernetes resources (but cannot mutate values).

• Architecture: Kyverno is a standalone admission controller that runs within the Kubernetes cluster. In contrast, Gatekeeper is a Kubernetes controller that runs outside the cluster, as a separate process.

[Ali-Yazdani]

I'm thinking about shaping this information in a table format. (I'm thinking loudly 🤔 )

[Ali-Yazdani]

@elgalu Please check the table.

[elgalu]

I would omit entries for which they all share the same properties, e.g. all 3 written in Go so doesn't help for choosing one over the other.

[elgalu]

So Gatekeeper actually also supports mutating, good to know. Found: Gatekeeper has (2 years ago) introduced the ability to mutate resources.