Document replacing illustration hex values with design tokens

What does this MR do?

Completes the <todo> in the illustration export guide and documents how to export and update product illustrations, including replacing hex values with --gl-illustration-* design tokens so illustrations adapt to dark mode. Related docs and the "New illustration" issue template now point to the guide.

Does this MR meet the acceptance criteria?

• The “What does this MR do?” section in the MR description is filled out, explaining the reasons for and scope of the proposed changes, per “Say why not just what”.
• Relevant label(s) are applied to the MR.
• The MR is added to a milestone.
• Content follows the GitLab Documentation Style Guide when appropriate.
• Related pages are cross-linked, where helpful.
• Review requested from any GitLab designer or directly from a maintainer or trainee maintainer.
• Maintainer review follows the Pajamas UX maintainer review checklist.

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