docs: clarify Global DR network requirements

[chinameok]

What changed

• Backport the Global DR network-requirement clarification from docs: clarify Global DR network requirements #969 to release-4.3.
• Document the bidirectional inter-cluster rules for TCP ports 443, 6443, 11443, and 2379, plus conditional port 80 when platform HTTP access is enabled.
• Explain why the rules must work in both directions after a failover swaps the primary and standby roles.
• Link the installation prerequisites port-forwarding section to the detailed Global DR network requirements.
• Use the official plugin name, Alauda Container Platform etcd Synchronizer, in customer-facing descriptions.

Why

AIT-72814 reported that the documentation did not make the required primary/standby network connectivity clear. The platform domain normally resolves to the primary cluster, so the standby reaches primary services through that domain. After failover, the former primary becomes the standby and requires the same connectivity toward the new primary.

Where to backport

• release-4.3 only.
• release-4.2, release-4.1, and release-4.0 are intentionally out of scope and can be handled later if users report the same gap.

Back-port Notes

• Cherry-picked with -x from master merge commit fe47e36fe217c9f8cb1803a5723faa7617fce627 (docs: clarify Global DR network requirements #969).
• Preserved the release-4.3 dependency baseline; no package or lockfile changes are included.
• Validation: yarn lint docs/en/install completed with 0 errors and 0 warnings; git diff --check passed.