Ensure DCO Workflow Check

[peternied]

Coming from opensearch-project/project-meta#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

[CEHENKLE]

We're good on both accounts. https://github.com/opensearch-project/opensearch-infra/tree/main/jenkins/jobs/OpenSearch_CI/PR_Checks :)

[dblock]

I'm reopening this one for DCO. The DCO check we use comes out of private Jenkins CI. I propose we nuke that and replace it with an OSS GHA workflow that runs on all branches here.