operator: add Kopf framework to list of libraries

`Kopf` is a python operator framework that is also well known and used.

Author: Frankkkkk

content/en/docs/concepts/extend-kubernetes/operator.md

@@ -111,6 +111,7 @@ Operator.
 {{% thirdparty-content %}}
 
 * [Charmed Operator Framework](https://juju.is/)
+* [Kopf](https://github.com/nolar/kopf) (Kubernetes Operator Pythonic Framework)
 * [kubebuilder](https://book.kubebuilder.io/)
 * [KubeOps](https://buehler.github.io/dotnet-operator-sdk/) (.NET operator SDK)
 * [KUDO](https://kudo.dev/) (Kubernetes Universal Declarative Operator)