Add anti-bot detection, retry, and fallback system

Automatically detect when crawls are blocked by anti-bot systems
(Akamai, Cloudflare, PerimeterX, DataDome, Imperva, etc.) and
escalate through configurable retry and fallback strategies.

New features on CrawlerRunConfig:
- max_retries: retry rounds when blocking is detected
- fallback_proxy_configs: list of fallback proxies tried each round
- fallback_fetch_function: async last-resort function returning raw HTML

New field on ProxyConfig:
- is_fallback: skip proxy on first attempt, activate only when blocked

Escalation chain per round: main proxy → fallback proxies in order.
After all rounds: fallback_fetch_function as last resort.

Detection uses tiered heuristics — structural HTML markers (high
confidence) trigger on any page, generic patterns only on short
error pages to avoid false positives.

mkdocs.yml

@@ -47,6 +47,7 @@ nav:
     - "Lazy Loading": "advanced/lazy-loading.md"
     - "Hooks & Auth": "advanced/hooks-auth.md"
     - "Proxy & Security": "advanced/proxy-security.md"
+    - "Anti-Bot & Fallback": "advanced/anti-bot-and-fallback.md"
     - "Undetected Browser": "advanced/undetected-browser.md"
     - "Session Management": "advanced/session-management.md"
     - "Multi-URL Crawling": "advanced/multi-url-crawling.md"