Add Datadog icon

Purpose

We're adding the logo to use for Integrations purposes. We are following https://www.datadoghq.com/about/resources/

Screenshots

Related issues

• https://gitlab.com/gitlab-com/legal-and-compliance/-/issues/2523
• gitlab#367605 (closed)

Checklist

For a new icon and/or illustration:

• Link to related design issue: gitlab#367605 (closed)
• If this adds an icon ensure that it follows the new icon checklists.
• If this adds an illustration ensure that it follows the new illustration checklists.

For third-party trademarks:

• If this adds or updates an icon or illustration that includes a third-party trademark, ensure that it follows the process for adding third-party trademarks to GitLab.