ci(docs-cn): migrate hub.pingcap.net images to cloud-accessible registries

[Copilot]

hub.pingcap.net is an internal IDC registry not reachable from cloud environments. The pingcap/docs-cn pipeline pod specs referenced two images from it.

Image replacements

Old	New
hub.pingcap.net/jenkins/docs-cn-checker:v1.0.0	us-docker.pkg.dev/pingcap-testing-account/hub/jenkins/docs-cn-checker:v1.0.0
hub.pingcap.net/jenkins/network-multitool	wbitt/network-multitool

Files changed

• pipelines/pingcap/docs-cn/latest/pod-merged_update_docs_cn.yaml — runner (docs-cn-checker) and net-tool containers
• pipelines/pingcap/docs-cn/latest/pod-pull_verify.yaml — net-tool container

Follows the same migration pattern as the pingcap/docs-tidb-operator pipeline (PR #4321): GAR mirror (us-docker.pkg.dev/pingcap-testing-account/hub) for internal custom images, public Docker Hub for generic utilities.

Original prompt

---

This section details on the original issue you should resolve

<issue_title>hub registry: migrate images for pingcap/docs-cn</issue_title>
<issue_description>Replace hub.pingcap.net image pulls/pushes for pingcap/docs-cn with a cloud-accessible registry.

Acceptance criteria:

• No hub.pingcap.net references in jobs/pipelines for pingcap/docs-cn
• Image registry endpoints documented and validated
  </issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes hub registry: migrate images for pingcap/docs-cn #4231

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