Write a document for Workload Completion

[qiujian16]

Add the workload completion document for manifestworks

design doc: https://github.com/open-cluster-management-io/enhancements/tree/main/enhancements/sig-architecture/138-workload-completion

doc to update: https://github.com/open-cluster-management-io/open-cluster-management-io.github.io/blob/main/content/en/docs/concepts/work-distribution/manifestwork.md