docs: migrate github-trending SDK cookbook to v2 API (2/N)

- Client -> ScrapeGraphAI (auto-reads SGAI_API_KEY)
- smartscraper(website_url=, user_prompt=, output_schema=)
  -> extract(prompt, url=, schema=model_json_schema())
- Response shape: response['result'] -> response.data.json_data
- Fix broken async example link
- Update dashboard URL: dashboard.scrapegraphai.com -> scrapegraphai.com/dashboard
- Swap outdated banner image

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: Vikrant-Khedkar