add placement cel examples

Signed-off-by: Qing Hao <[email protected]>

Author: haoqing0110

content/en/docs/concepts/content-placement/placement.md

@@ -62,7 +62,10 @@ sets in the hub clusters. Then we can move on and create a placement in the
 
 #### Label/Claim selection
 
-In the `predicates` section, you can select clusters by labels or [clusterClaims](../cluster-inventory/clusterclaim).
+In the `predicates` section, you can select clusters by labels, [clusterClaims](../cluster-inventory/clusterclaim), or CEL expressions. 
+
+1. Using labels and clusterClaims:
+
 For instance, you can select 3 clusters with label `purpose=test` and
 clusterClaim `platform.open-cluster-management.io=aws` as seen in the following
 examples:
@@ -72,7 +75,7 @@ apiVersion: cluster.open-cluster-management.io/v1beta1
 kind: Placement
 metadata:
   name: placement1
-  namespace: default
+  namespace: ns1
 spec:
   numberOfClusters: 3
   clusterSets:
@@ -94,6 +97,39 @@ Note that the distinction between label-selecting and claim-selecting is
 elaborated in [this page]({{< ref "docs/scenarios/extending-managed-clusters" >}})
 about how to extend attributes for the managed clusters.
 
+2. Using CEL expressions:
+
+```yaml
+apiVersion: cluster.open-cluster-management.io/v1beta1
+kind: Placement
+metadata:
+  name: placement1
+  namespace: ns1
+spec:
+  numberOfClusters: 3
+  clusterSets:
+    - prod
+  predicates:
+  - requiredClusterSelector:
+      celSelector:
+        celExpressions:
+          # Select clusters by Kubernetes version listed in managedCluster.Status.version.kubernetes.
+          - managedCluster.status.version.kubernetes == "v1.31.0"
+          # Select clusters by info stored in clusterClaims.
+          - managedCluster.status.clusterClaims.exists(c, c.name == "kubeversion.open-cluster-management.io" && c.value == "v1.31.0")
+          # Use CEL Standard macros and Standard functions on the managedCluster fields.
+          - managedCluster.metadata.labels["version"].matches('^1\\.(30|31)\\.\\d+$')
+          # Use Kubernetes semver library functions isLessThan and isGreaterThan to select clusters by version comparison. 
+          - semver(managedCluster.metadata.labels["version"]).isGreaterThan(semver("1.30.0")) 
+          # Use OCM customized function scores to select clusters by AddonPlacementScore.
+          - managedCluster.scores("resource-usage-score").filter(s, s.name == 'memNodeAvailable').all(e, e.value > 0)
+```
+
+The CEL expressions provide more flexible and powerful selection capabilities with [built-in libraries](https://github.com/open-cluster-management-io/sdk-go/blob/main/pkg/cel/common/values.go). For more detailed usage of CEL expressions, refer to:
+
+- [Kubernetes CEL Documentation](https://kubernetes.io/docs/reference/using-api/cel/)
+- [OCM CEL Library](https://github.com/open-cluster-management-io/sdk-go/tree/main/pkg/cel/library)
+
 #### Taints/Tolerations
 
 To support filtering unhealthy/not-reporting clusters and keep workloads from
@@ -133,7 +169,7 @@ The following example shows how to tolerate clusters with taints.
           timeAdded: '2022-02-21T08:11:06Z'
     ```
 
-    By default, the placement won’t select this cluster unless you define tolerations.
+    By default, the placement won't select this cluster unless you define tolerations.
 
     ```yaml
     apiVersion: cluster.open-cluster-management.io/v1beta1