Add Diffblue logo

Purpose

We are adding Asana integration logos, as per gitlab#435170 (closed).

The icon has been importer from https://www.diffblue.com/brand/, and per brand guidance:

When embedding our logo in an article, we prefer the full logo to be used over the logomark.

Screenshots

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.