feat: revise getting started experience with framework-first IA

[chrisraygill]

• Add "Choose your stack" hub page as primary onboarding entry point
• Restructure sidebar: lean "Get started" section, new "Frameworks" section with language filtering, "Client SDKs" replaced by Web client and Flutter pages
• Add framework guides for Next.js App Router, Next.js Pages, Hono, Fastify, Gin, Echo, Chi, Django, and Web client
• Make Flutter guide language-agnostic (works with any Genkit backend)
• Add "Work with Genkit in AI tools" page for MCP/IDE setup
• Add CORS handling to Go framework guides (gin-contrib/cors, Echo middleware, go-chi/cors)
• Fix Output import bug in FastAPI and Flask guides (use output_schema= instead of Output())
• Ignore .scratch/ directory

[gemini-code-assist]

Code Review

This pull request significantly restructures the Genkit documentation by introducing a "Choose your stack" entry point and adding or rewriting integration guides for various frameworks across JavaScript, Go, Python, and Dart. Key changes include new guides for Next.js (App and Pages routers), Fastify, Hono, Django, Gin, Echo, Chi, and a generic Web Client, alongside simplified versions of existing guides for Angular, Express, FastAPI, Flask, Flutter, and Shelf. Feedback focuses on correcting a recurring typo where gemini-2.5-flash was used instead of gemini-2.0-flash across multiple files, and improving request cancellation logic in the Fastify guide.

[gemini-code-assist]

Using AbortSignal.abort() only provides a signal that is already aborted. To correctly support request cancellation, you should pass the request's own signal.

    signal: request.signal,