Add Discord logo

Purpose

We are adding Discord integration logo, as per gitlab#367604, followed the spreadsheets.

Approval status:

"No agreement was found on file in the account. If there's an agreement in place, it will need to be reviewed by Legal before proceeding. Please confirm whether or not a current agreement exists. If there's no written agreement with Discord, then the logo can be used as long as their brand guidelines are followed. "

note: we already had approved Discord logo, but it was located under the sprite_icons, here we're grouping all integrations' logos together.

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.