docs: add generative UI explainer article

[nicovaleops]

Closes #7

Summary

• Adds a developer-focused article explaining generative UI and why plain text output is no longer enough for stateful, reviewable AI workflows.
• Covers the practical UI contract: structured state, visible controls, validation, streaming progress, and reusable components.
• Positions OpenUI as a way to turn model output into inspectable interfaces instead of one-off prose.

Validation

• Private/internal term scan passed.
• npx --yes markdownlint-cli "Articles/generative-ui-text-output-not-enough.md"
• git diff --check