docs: Add "KubeOps" operator SDK to third-party list

Adding dotnet operator sdk/framework to third-party list of operator sdks.

Author: buehler

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

@@ -119,6 +119,7 @@ Operator.
   you implement yourself
 * [Operator Framework](https://operatorframework.io)
 * [shell-operator](https://github.com/flant/shell-operator)
+* [KubeOps](https://buehler.github.io/dotnet-operator-sdk/)
 
 ## {{% heading "whatsnext" %}}