docs(setup): document global.networkPolicy configuration

[tamalsaha]

Summary

Documents the global.networkPolicy Helm values introduced in kubestash/installer#348, which adds Cilium support alongside the built-in Kubernetes NetworkPolicy.

Adds a Network Policy section to the KubeStash install configuration docs:

• How to enable it (enabled: true) and select the flavor (kubernetes vs cilium).
• A --set example for inline enablement.
• A Required network communication subsection listing the in-cluster paths the generated policies allow (operator → kube-apiserver, operator → DNS, kube-apiserver → webhook server), plus a note on backup/restore job connectivity.

Test plan

• Docs render correctly (Markdown + Hugo shortcodes).

[github-actions]

Visit the preview URL for this PR (updated for commit ac3e98b):

https://kubestash-qa--pr88-docs-network-policy-qkrqgb22.web.app

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