Add DEI.md file for main GitLab project

What does this MR do and why?

This MR adds a DEI.md file into the GitLab project as part of the CHAOSS DEI Project Badging initiative.

GitLab has partnered with CHAOSS, a Linux foundation project, to help projects showcase their commitment to improving and supporting diversity, equity and inclusion.

This MR is the first step for GitLab to be eligible to earn a DEI Badge from CHAOSS. GitLab will first seek review for the main GitLab project from CHAOSS before adding the DEI.md file into other GitLab projects. GitLab will then seek to get open source projects hosted on GitLab to apply for the DEI Badge.

Helpful links:

• Issue: gitlab-org/developer-relations/contributor-success/team-task#287 (closed)
• Guide to making the DEI.md file: https://github.com/badging/ProjectBadging/blob/main/Guide.DEI.md
• Template for making the DEI.md file: https://github.com/badging/ProjectBadging/blob/main/Template.DEI.md
• Example of an existing DEI.md file from CHAOSS: https://github.com/chaoss/community/blob/main/DEI.md

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

Before	After

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.