Updates Figma page for marketing team

Why is this change being made?

💡 Provide a detailed answer to the question on why this change is being proposed, in accordance with our value of Transparency.

I was reviewing the Figma resources for Product Designer Onboarding as part of gitlab-org/gitlab-design!368 (comment 1617144538) and found this page. I think it should be made clear that this is the process for the marketing designers and not the product designers because we don't follow this guidance.

Also, the Loom video demo at the bottom is broken so needs to be replaced or removed altogether.

Author and Reviewer Checklist

Please verify the check list and ensure to tick them off before the MR is merged.

• Provided a concise title for this Merge Request (MR)
• Added a description to this MR explaining the reasons for the proposed change, per say why, not just what
  • Copy/paste the Slack conversation to document it for later, or upload screenshots. Verify that no confidential data is added, and the content is SAFE
• Assign reviewers for this MR to the correct Directly Responsible Individual/s (DRI)
  • If the DRI for the page/s being updated isn’t immediately clear, then assign it to one of the people listed in the Maintained by section on the page being edited
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  • The when to get approval handbook section explains the workflow in more detail
• For transparency, share this MR with the audience that will be impacted.
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