Adding of Developer Certificate of Origin to docs

What does this MR do?

This MR adds GitLab's Developer Certificate of Origin (DCO) to GitLab Documentation.

Contributions from the open source community are made subject to three different agreements, depending on the circumstances:

• contributions made on behalf of an individual to the ee/ directory are subject to the Individual Contributor License Agreement

• contributions made on behalf of a company to the ee/ directory are subject to the Corporate Contributor License Agreement

• contributions made outside of the ee/ directory are subject to the Developer Certificate of Origin

The Individual and Corporate CLAs are already included in GitLab Documentation. For the sake of consistency, and to provide an SSoT for the three contribution agreements, this MR adds the DCO to GitLab Documentation.