docs: Add docs for cel based Validating Admission Policy support

website/docs/validating-admission-policy

@@ -0,0 +1,71 @@
+---
+id: validating-admission-policy
+title: Integration with Kubernetes Validating Admission Policy
+---
+
+`Feature State`: Gatekeeper version v3.13+ (pre-alpha)
+
+> ❗ This feature is pre-alpha, subject to change (feedback is welcome!). It is disabled by default. To enable the feature,
+> set the `experimental-enable-k8s-native-validation` flag to true and use the [development build of Gatekeeper](https://open-policy-agent.github.io/gatekeeper/website/docs/install/#deploying-a-release-using-development-image).
+
+## Description
+
+This feature allows Gatekeeper to integrate with Kubernetes Validating Admission Policy based on [Common Expression Language (CEL)](https://github.com/google/cel-spec), a declarative, in-process admission control alternative to validating admission webhooks.
+
+## Motivations
+
+The Validating Admission Policy feature (disabled by default) was introduced as an alpha feature to Kubernetes v1.26, beta in v1.28. Some of the benefits include:
+- in-tree/native in-process
+- reduce admission request latency
+- improve reliability and availability
+- able to fail closed without impacting availability
+- avoid the operational burden of webhooks
+
+To reduce policy fragmentation and simplify the user experience by standardizing the policy experience. We have created an abstraction layer that provides multi-language (e.g. Rego and CEL), multi-target policy enforcement to allow for portable policies and coexistence of numerous policy implementations.
+
+The [Constraint Framework](https://github.com/open-policy-agent/frameworks/tree/master/constraint) is the library that underlies Gatekeeper. It provides the execution flow Gatekeeper uses to render a decision to the API server. It also provides abstractions that allow us to define constraint templates and constraints: Engine, Enforcement Points, and Targets.
+
+Together with Gatekeeper and [gator CLI](gator.md), you can get admission, audit, and shift left validations for both CEL-based Validating Admission Policy and OPA Rego policies, even for clusters that do not support Validating Admission Policy feature yet.
+
+## Example Constraint Template
+To see how it works, check out this [demo](https://github.com/open-policy-agent/gatekeeper/tree/master/demo/k8s-validating-admission-policy)
+
+Example `K8sRequiredLabels` constraint template using the `K8sNativeValidation` engine and CEL expressions that requires resources to contain specified labels with values matching provided regular expressions. A similar policy written in Rego can be seen [here](https://open-policy-agent.github.io/gatekeeper-library/website/validation/requiredlabels)
+
+```yaml
+apiVersion: templates.gatekeeper.sh/v1
+kind: ConstraintTemplate
+metadata:
+  name: k8srequiredlabels
+spec:
+  crd:
+    spec:
+      names:
+        kind: K8sRequiredLabels
+      validation:
+        # Schema for the `parameters` field
+        openAPIV3Schema:
+          type: object
+          properties:
+            message:
+              type: string
+            labels:
+              type: array
+              items:
+                type: object
+                properties:
+                  key:
+                    type: string
+                  allowedRegex:
+                    type: string
+  targets:
+    - target: admission.k8s.gatekeeper.sh
+      code:
+        - engine: K8sNativeValidation
+          source:
+            validations:
+            - expression: "variables.params.labels.all(entry, has(object.metadata.labels) && entry.key in object.metadata.labels)"
+              messageExpression: '"missing required label, requires all of: " + variables.params.labels.map(entry, entry.key).join(", ")'
+            - expression: "!variables.params.labels.exists(entry, has(object.metadata.labels) && entry.key in object.metadata.labels && !string(object.metadata.labels[entry.key]).matches(string(entry.allowedRegex)))"
+              message: "regex mismatch"
+```

website/sidebars.js

@@ -34,7 +34,8 @@ module.exports = {
         'expansion',
         'gator',
         'workload-resources',
-        'pubsub'
+        'pubsub',
+        'validating-admission-policy'
       ],
     },
     {

website/versioned_docs/version-v3.13.x/validating-admission-policy

@@ -0,0 +1,71 @@
+---
+id: validating-admission-policy
+title: Integration with Kubernetes Validating Admission Policy
+---
+
+`Feature State`: Gatekeeper version v3.13+ (pre-alpha)
+
+> ❗ This feature is pre-alpha, subject to change (feedback is welcome!). It is disabled by default. To enable the feature,
+> set the `experimental-enable-k8s-native-validation` flag to true and use the [development build of Gatekeeper](https://open-policy-agent.github.io/gatekeeper/website/docs/install/#deploying-a-release-using-development-image).
+
+## Description
+
+This feature allows Gatekeeper to integrate with Kubernetes Validating Admission Policy based on [Common Expression Language (CEL)](https://github.com/google/cel-spec), a declarative, in-process admission control alternative to validating admission webhooks.
+
+## Motivations
+
+The Validating Admission Policy feature (disabled by default) was introduced as an alpha feature to Kubernetes v1.26, beta in v1.28. Some of the benefits include:
+- in-tree/native in-process
+- reduce admission request latency
+- improve reliability and availability
+- able to fail closed without impacting availability
+- avoid the operational burden of webhooks
+
+To reduce policy fragmentation and simplify the user experience by standardizing the policy experience. We have created an abstraction layer that provides multi-language (e.g. Rego and CEL), multi-target policy enforcement to allow for portable policies and coexistence of numerous policy implementations.
+
+The [Constraint Framework](https://github.com/open-policy-agent/frameworks/tree/master/constraint) is the library that underlies Gatekeeper. It provides the execution flow Gatekeeper uses to render a decision to the API server. It also provides abstractions that allow us to define constraint templates and constraints: Engine, Enforcement Points, and Targets.
+
+Together with Gatekeeper and [gator CLI](gator.md), you can get admission, audit, and shift left validations for both CEL-based Validating Admission Policy and OPA Rego policies, even for clusters that do not support Validating Admission Policy feature yet.
+
+## Example Constraint Template
+To see how it works, check out this [demo](https://github.com/open-policy-agent/gatekeeper/tree/master/demo/k8s-validating-admission-policy)
+
+Example `K8sRequiredLabels` constraint template using the `K8sNativeValidation` engine and CEL expressions that requires resources to contain specified labels with values matching provided regular expressions. A similar policy written in Rego can be seen [here](https://open-policy-agent.github.io/gatekeeper-library/website/validation/requiredlabels)
+
+```yaml
+apiVersion: templates.gatekeeper.sh/v1
+kind: ConstraintTemplate
+metadata:
+  name: k8srequiredlabels
+spec:
+  crd:
+    spec:
+      names:
+        kind: K8sRequiredLabels
+      validation:
+        # Schema for the `parameters` field
+        openAPIV3Schema:
+          type: object
+          properties:
+            message:
+              type: string
+            labels:
+              type: array
+              items:
+                type: object
+                properties:
+                  key:
+                    type: string
+                  allowedRegex:
+                    type: string
+  targets:
+    - target: admission.k8s.gatekeeper.sh
+      code:
+        - engine: K8sNativeValidation
+          source:
+            validations:
+            - expression: "variables.params.labels.all(entry, has(object.metadata.labels) && entry.key in object.metadata.labels)"
+              messageExpression: '"missing required label, requires all of: " + variables.params.labels.map(entry, entry.key).join(", ")'
+            - expression: "!variables.params.labels.exists(entry, has(object.metadata.labels) && entry.key in object.metadata.labels && !string(object.metadata.labels[entry.key]).matches(string(entry.allowedRegex)))"
+              message: "regex mismatch"
+```

website/versioned_sidebars/version-v3.13.x-sidebars.json

@@ -34,7 +34,8 @@
         "expansion",
         "gator",
         "workload-resources",
-        "pubsub"
+        "pubsub",
+        "validating-admission-policy"
       ]
     },
     {