Summary: Cosmo app launch

Why It Matters: This change shows CodeSignal Learn now has a mobile app.

What Changed:

• Removed "Popular topics" section header.
• Removed links to "Learn to Code," "Soft Skills," "Data Science & Eng," "Interview Prep," "AI & Machine Learning," and "Web Development."
• Added a new section header: "Your favorite way to learn, now on mobile."
• Added text: "Turn screen time into skills time with the Cosmo app. Download for free!"
• Added two image links, likely for app store downloads.