refactor(dispatcher): migrate to modular dispatcher system with enhanced monitoring

Reorganize dispatcher functionality into separate components:
- Create dedicated dispatcher classes (MemoryAdaptive, Semaphore)
- Add RateLimiter for smart request throttling
- Implement CrawlerMonitor for real-time progress tracking
- Move dispatcher config from CrawlerRunConfig to separate classes

BREAKING CHANGE: Dispatcher configuration moved from CrawlerRunConfig to dedicated dispatcher classes. Users need to update their configuration approach for multi-URL crawling.

docs/md_v2/api/arun.md

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-Below is a **revised parameter guide** for **`arun()`** in **AsyncWebCrawler**, reflecting the **new** approach where all parameters are passed via a **`CrawlerRunConfig`** instead of directly to `arun()`. Each section includes example usage in the new style, ensuring a clear, modern approach.
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 # `arun()` Parameter Guide (New Approach)
 
 In Crawl4AI’s **latest** configuration model, nearly all parameters that once went directly to `arun()` are now part of **`CrawlerRunConfig`**. When calling `arun()`, you provide: