Add GitLab Transcend event promotion to What's New drawer

What does this MR do and why?

Add a card to the What's New drawer promoting the GitLab Transcend event. The entry should link users out to the marketing site registration page.

References

Screenshots or screen recordings

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How to set up and validate locally

rails console

> Feature.enable(:whats_new_transcend_promo)

• Setup gdk to simulate SaaS - https://docs.gitlab.com/ee/development/ee_features.html#simulate-a-saas-instance and restart GDK
• Sign in. Open What's new from the Help menu.
• Click Register now
• Verify click_whats_new_transcend_register frontend event is triggered.

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Related to #600482