Add the Figma plugin link in the icon issue template

Purpose

Add a tip to use the icon plugin in the issue template. It will help designers not to run multiple pipelines while adding a new icon to the library.

• Related discussion in the MR: !1029 (comment 1753637055)

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.