Add generative UI explainer article

[Naustudents53]

Summary

• Adds a developer-focused explainer for issue Written Content: What Is Generative UI? (And Why Text Output Is No Longer Enough) #7: "What Is Generative UI?"
• Covers why text-only AI output breaks down, what generative UI means in practice, how it differs from templates/chatbots, and the architecture behind component-constrained UI generation
• Includes OpenUI naturally as one concrete implementation path and links references at the end

Notes

Submitted as Markdown for the OpenUI Creator Program. Happy to revise tone, depth, examples, or structure during review.