Update doc for PNI in GKE

[krunalhinguu]

Fixes #<add_issue_number_if_any>

Description

• Added a note under Project Network Isolation in the GKE cluster configuration page.
• Clarifies that for imported GKE clusters, Network Policy (Calico) must be enabled on both master and worker nodes before enabling PNI.

Comments

• This update improves clarity for users importing GKE clusters with PNI.

[btat]

Suggested change

	choose whether to enable or disable inter-project communication.
	Choose whether to enable or disable inter-project communication.

[btat]

Suggested change

	:::note
	For **imported GKE clusters**, Project Network Isolation (PNI) requires Kubernetes Network Policy to be enabled on the cluster beforehand.
	- Rancher will enable Network Policy automatically when creating clusters in Rancher (downstream), so this step is only needed for imported clusters.
	- In GKE, enable network policy (Calico) on **both master and worker nodes**: **Networking → Network security and observability → Enable Calico Kubernetes Network Policy**.
	- After enabling, import the cluster into Rancher and enable PNI for project-level isolation.
	:::
	#### Imported Clusters
	For imported clusters, Project Network Isolation (PNI) requires enabling Kubernetes Network Policy on the cluster beforehand. For clusters created by Rancher, Rancher automatically enables Kubernetes Network Policy.
	1. In GKE, enable network policy (Calico) on **both master and worker nodes**
	1. Select the cluster to enable network policy on.
	1. **Networking → Network security and observability → Enable Calico Kubernetes Network Policy**.
	1. In Rancher, import the cluster and enable PNI for project-level isolation.

Thoughts on converting it into a subsection so that it can be linked if needed?

Some questions:

• Is **both master and worker nodes** a field that appears in the GKE console or is it intended to be text telling users they need to do it for both?
• For the steps done in GKE, are we replicating these instructions? If so, maybe we could link to that to reduce maintenance if upstream changes anything.

[krunalhinguu]

Answering your questions

1. Both master and worker nodes is not a field in the GKE console. GKE only has one option to enable Network Policy, and when you turn it on, GKE applies it to the whole cluster automatically. I removed that wording to avoid confusion.
2. agree, we should link instead of duplicating exact UI steps