Ensure DCO Workflow Check

[peternied]

Coming from https://github.com/opensearch-project/project-meta/issues/17

A Developer Certificate of Origin is required on commits in the OpenSearch-Project.

See doc.yml for an example workflow. Ensure CONTRIBUTING.md to has a section on the DCO per the project template.

• [ ] DCO Check Workflow
• [ ] CONTRIBUTING.md DCO Section