feat(crawl4ai): Implement adaptive crawling feature

This commit introduces the adaptive crawling feature to the crawl4ai project. The adaptive crawling feature intelligently determines when sufficient information has been gathered during a crawl, improving efficiency and reducing unnecessary resource usage.

The changes include the addition of new files related to the adaptive crawler, modifications to the existing files, and updates to the documentation. The new files include the main adaptive crawler script, utility functions, and various configuration and strategy scripts. The existing files that were modified include the project's initialization file and utility functions. The documentation has been updated to include detailed explanations and examples of the adaptive crawling feature.

The adaptive crawling feature will significantly enhance the capabilities of the crawl4ai project, providing users with a more efficient and intelligent web crawling tool.

Significant modifications:
- Added adaptive_crawler.py and related scripts
- Modified __init__.py and utils.py
- Updated documentation with details about the adaptive crawling feature
- Added tests for the new feature

BREAKING CHANGE: This is a significant feature addition that may affect the overall behavior of the crawl4ai project. Users are advised to review the updated documentation to understand how to use the new feature.

Refs: #123, #456

docs/examples/adaptive_crawling/embedding_strategy.py

@@ -0,0 +1,109 @@
+"""
+Embedding Strategy Example for Adaptive Crawling
+
+This example demonstrates how to use the embedding-based strategy
+for semantic understanding and intelligent crawling.
+"""
+
+import asyncio
+import os
+from crawl4ai import AsyncWebCrawler, AdaptiveCrawler, AdaptiveConfig
+
+
+async def main():
+    """Demonstrate embedding strategy for adaptive crawling"""
+    
+    # Configure embedding strategy
+    config = AdaptiveConfig(
+        strategy="embedding",  # Use embedding strategy
+        embedding_model="sentence-transformers/all-MiniLM-L6-v2",  # Default model
+        n_query_variations=10,  # Generate 10 semantic variations
+        max_pages=15,
+        top_k_links=3,
+        min_gain_threshold=0.05,
+        
+        # Embedding-specific parameters
+        embedding_k_exp=3.0,  # Higher = stricter similarity requirements
+        embedding_min_confidence_threshold=0.1,  # Stop if <10% relevant
+        embedding_validation_min_score=0.4  # Validation threshold
+    )
+    
+    # Optional: Use OpenAI embeddings instead
+    if os.getenv('OPENAI_API_KEY'):
+        config.embedding_llm_config = {
+            'provider': 'openai/text-embedding-3-small',
+            'api_token': os.getenv('OPENAI_API_KEY')
+        }
+        print("Using OpenAI embeddings")
+    else:
+        print("Using sentence-transformers (local embeddings)")
+    
+    async with AsyncWebCrawler(verbose=True) as crawler:
+        adaptive = AdaptiveCrawler(crawler, config)
+        
+        # Test 1: Relevant query with semantic understanding
+        print("\n" + "="*50)
+        print("TEST 1: Semantic Query Understanding")
+        print("="*50)
+        
+        result = await adaptive.digest(
+            start_url="https://docs.python.org/3/library/asyncio.html",
+            query="concurrent programming event-driven architecture"
+        )
+        
+        print("\nQuery Expansion:")
+        print(f"Original query expanded to {len(result.expanded_queries)} variations")
+        for i, q in enumerate(result.expanded_queries[:3], 1):
+            print(f"  {i}. {q}")
+        print("  ...")
+        
+        print("\nResults:")
+        adaptive.print_stats(detailed=False)
+        
+        # Test 2: Detecting irrelevant queries
+        print("\n" + "="*50)
+        print("TEST 2: Irrelevant Query Detection")
+        print("="*50)
+        
+        # Reset crawler for new query
+        adaptive = AdaptiveCrawler(crawler, config)
+        
+        result = await adaptive.digest(
+            start_url="https://docs.python.org/3/library/asyncio.html",
+            query="how to bake chocolate chip cookies"
+        )
+        
+        if result.metrics.get('is_irrelevant', False):
+            print("\n✅ Successfully detected irrelevant query!")
+            print(f"Stopped after just {len(result.crawled_urls)} pages")
+            print(f"Reason: {result.metrics.get('stopped_reason', 'unknown')}")
+        else:
+            print("\n❌ Failed to detect irrelevance")
+        
+        print(f"Final confidence: {adaptive.confidence:.1%}")
+        
+        # Test 3: Semantic gap analysis
+        print("\n" + "="*50)
+        print("TEST 3: Semantic Gap Analysis")
+        print("="*50)
+        
+        # Show how embedding strategy identifies gaps
+        adaptive = AdaptiveCrawler(crawler, config)
+        
+        result = await adaptive.digest(
+            start_url="https://realpython.com",
+            query="python decorators advanced patterns"
+        )
+        
+        print(f"\nSemantic gaps identified: {len(result.semantic_gaps)}")
+        print(f"Knowledge base embeddings shape: {result.kb_embeddings.shape if result.kb_embeddings is not None else 'None'}")
+        
+        # Show coverage metrics specific to embedding strategy
+        print("\nEmbedding-specific metrics:")
+        print(f"  Average best similarity: {result.metrics.get('avg_best_similarity', 0):.3f}")
+        print(f"  Coverage score: {result.metrics.get('coverage_score', 0):.3f}")
+        print(f"  Validation confidence: {result.metrics.get('validation_confidence', 0):.2%}")
+
+
+if __name__ == "__main__":
+    asyncio.run(main())