feat/add Firecrawl backend support to crawler

[Akeemkabiru]

I introduces Firecrawl as an optional backend for crawl4ai.

Updates

• Added FirecrawlBackend wrapper around Firecrawl’s SDK.
• Extended CLI with --backend option (default | firecrawl).
• Enabled output in multiple formats (json, markdown, markdown-fit).
• Added a standalone script (firecrawl_backend.py) showing how to use Firecrawl programmatically.
• Updated README with installation and usage instructions for Firecrawl.

Example usage : python -m crawl4ai.cli crawl https://docs.firecrawl.dev --backend firecrawl --output markdown