Override docs use v1alpha1 but types exist in v1beta1 and v1

[ytimocin]

The following docs use apiVersion: placement.kubernetes-fleet.io/v1alpha1 for ClusterResourceOverride and ResourceOverride:

• content/en/docs/how-tos/cluster-resource-override.md
• content/en/docs/how-tos/resource-override.md
• content/en/docs/concepts/override.md

In the kubefleet source, ClusterResourceOverride and ResourceOverride are defined in both apis/placement/v1beta1/override_types.go and apis/placement/v1/override_types.go. The v1alpha1 types file (apis/placement/v1alpha1/override_types.go) imports directly from v1beta1 and exists only for backward compatibility. All three docs should be updated to use v1 (GA) API version in their YAML examples.