docs(falcon-ai): add Imagine feature page

[SuhaniNagpal7]

Summary

• New page: src/pages/docs/falcon-ai/features/imagine.mdx — covers what Imagine is (custom dashboard builder for traces), how to open it, build widgets via chat, save reusable views, and how dynamic analysis re-runs LLM widgets on every trace.
• Sidebar entry added in src/lib/navigation.ts under Falcon AI → Features (between Using Falcon AI and Skill Builder).
• 4 light-theme product screenshots captured at public/screenshot/product/falcon/10.png–13.png: empty canvas with suggested prompts, dashboard with widgets, Save View popover (cropped), and dynamic-analysis skeleton loader.

Test plan

• pnpm dev and verify the page renders at /docs/falcon-ai/features/imagine
• Sidebar shows Imagine under Falcon AI → Features
• All four screenshots load (no broken images)
• Internal links work: Dynamic analysis anchor, Next Steps cards to chat / skills / mcp-connectors
• No em dashes in prose; <Steps> and <Note> components render correctly