Document JiHu validation pipelines

Why is this change being made?

We need to let people know what to do when the JiHu validation pipelines failed

Review app: https://document-jh-validation-pipeline.about.gitlab-review.app/handbook/ceo/chief-of-staff-team/jihu-support/jihu-validation-pipelines.html

See gitlab-org/gitlab#390683 (closed)

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