docs(installation): clarify Helm requirement in Kubernetes setup

- Added note specifying Helm is required for installation as non-Helm
  installations are not supported currently.

Author: sabban

crowdsec-docs/unversioned/getting_started/installation/kubernetes.mdx

@@ -19,6 +19,8 @@ Before getting started, it is advised to read the [introduction](/unversioned/ge
 - [kubectl](https://kubernetes.io/docs/tasks/tools/#kubectl)
 - [Helm](https://helm.sh/docs/intro/install/)
 
+Even if an installation could be possible without Helm, it's not supported for now.
+
 ## Helm Repository Installation
 
 Add the CrowdSec helm repository to your Helm installation: