docs: clarify Global DR network requirements

[chinameok]

What changed

• Added explicit network prerequisites for the Primary and Standby Global Clusters.
• Documented the purpose of TCP ports 443, optional 80, 6443, 11443, and 2379.
• Added a direct link from the general LoadBalancer Forwarding Rules to the Global DR Network Requirements section.
• Clarified that the Standby Cluster uses the platform domain to access platform services on the Primary Cluster.
• Clarified that both cluster VIPs need the required listeners and that inter-cluster firewall, security-group, routing, and ACL rules must allow the ports in both directions.
• Used the official plugin name, Alauda Container Platform etcd Synchronizer, in customer-facing descriptions.
• Clarified that the network policy is bidirectional while etcd synchronization remains one-way from the current Primary to the current Standby.

Why

The platform domain normally resolves only to the Primary Cluster, so steady-state traffic is normally initiated from Standby to Primary. A failover swaps the cluster roles and reverses the required connection direction. The existing documentation listed the ports but did not explain this bidirectional network prerequisite, which could lead to a one-way configuration that fails after switchover. The cross-reference ensures that users configuring general load balancer rules discover the complete Global DR requirements before installation.

Where to backport

This PR targets master only. Do not backport yet. After the master PR is reviewed and merged, backport to applicable release branches only after separate authorization from Chao.

Validation

• yarn lint docs/en/install: 0 errors, 0 warnings
• Customer-facing terminology, internal-token, and EN-only checks passed

Review level

Technical correctness and customer-operable Global DR network prerequisites.

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