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

[Copilot]

Removes all hub.pingcap.net container image references from the pingcap/docs-tidb-operator pipeline, replacing them with publicly accessible equivalents.

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

• us-docker.pkg.dev/pingcap-testing-account/hub is the established GCP mirror for internal hub.pingcap.net images with no public equivalent
• wbitt/network-multitool is the public Docker Hub image already used across other pipelines and prow-jobs in this repo

Original prompt

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This section details on the original issue you should resolve

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

Acceptance criteria:

• No hub.pingcap.net references in jobs/pipelines for pingcap/docs-tidb-operator
• 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-tidb-operator #4232

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Hi @Copilot. Thanks for your PR.

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