Add "bidirectional-xs" illustration

Purpose

Add XS illustration to support #275 (closed).

Screenshots

The illustration is used to represent "integration" and is the small green circle with arrows between the two logos.

Checklist

For a new icon and/or illustration:

• Link to related design issue:
• 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.

Relates to #275 (closed)