refactor(core): reorganize project structure and remove legacy code

Major reorganization of the project structure:
- Moved legacy synchronous crawler code to legacy folder
- Removed deprecated CLI and docs manager
- Consolidated version manager into utils.py
- Added CrawlerHub to __init__.py exports
- Fixed type hints in async_webcrawler.py
- Fixed minor bugs in chunking and crawler strategies

BREAKING CHANGE: Removed synchronous WebCrawler, CLI, and docs management functionality. Users should migrate to AsyncWebCrawler.

crawl4ai/hub.py

@@ -0,0 +1,73 @@
+import importlib
+import pkgutil
+from pathlib import Path
+import logging
+from typing import Dict, Type
+import inspect
+
+logger = logging.getLogger(__name__)
+
+# crawl4ai/base.py
+from abc import ABC, abstractmethod
+from typing import Optional, Dict, Any
+import json
+import logging
+
+class BaseCrawler(ABC):
+    def __init__(self):
+        self.logger = logging.getLogger(self.__class__.__name__)
+        
+    @abstractmethod
+    async def run(self, url: str = "", **kwargs) -> str:
+        """
+        Implement this method to return JSON string.
+        Must accept URL + arbitrary kwargs for flexibility.
+        """
+        pass
+
+    def __init_subclass__(cls, **kwargs):
+        """Enforce interface validation on subclassing"""
+        super().__init_subclass__(**kwargs)
+        
+        # Verify run method signature
+        run_method = cls.run
+        if not run_method.__code__.co_argcount >= 2:  # self + url
+            raise TypeError(f"{cls.__name__} must implement 'run(self, url: str, **kwargs)'")
+            
+        # Verify async nature
+        if not inspect.iscoroutinefunction(run_method):
+            raise TypeError(f"{cls.__name__}.run must be async")
+
+class CrawlerHub:
+    _crawlers: Dict[str, Type[BaseCrawler]] = {}
+
+    @classmethod
+    def _discover_crawlers(cls):
+        """Dynamically load crawlers from /crawlers in 3 lines"""
+        base_path = Path(__file__).parent / "crawlers"
+        for crawler_dir in base_path.iterdir():
+            if crawler_dir.is_dir():
+                try:
+                    module = importlib.import_module(
+                        f"crawl4ai.crawlers.{crawler_dir.name}.crawler"
+                    )
+                    for attr in dir(module):
+                        cls._maybe_register_crawler(
+                            getattr(module, attr), crawler_dir.name
+                        )
+                except Exception as e:
+                    logger.warning(f"Failed {crawler_dir.name}: {str(e)}")
+
+    @classmethod
+    def _maybe_register_crawler(cls, obj, name: str):
+        """Brilliant one-liner registration"""
+        if isinstance(obj, type) and issubclass(obj, BaseCrawler) and obj != BaseCrawler:
+            module = importlib.import_module(obj.__module__)
+            obj.meta = getattr(module, "__meta__", {})
+            cls._crawlers[name] = obj
+
+    @classmethod
+    def get(cls, name: str) -> Type[BaseCrawler] | None:
+        if not cls._crawlers:
+            cls._discover_crawlers()
+        return cls._crawlers.get(name)