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.

docs/md_v2/advanced/proxy-security.md

@@ -302,3 +302,7 @@ def safe_proxy_repr(proxy: ProxyConfig):
     - Ensure `ProxyConfig.from_env()` actually loaded entries (`len(proxies) > 0`).
     - Attach `proxy_rotation_strategy` to `CrawlerRunConfig`.
     - Validate the proxy definitions you pass into the strategy.
+
+## See Also
+
+- [Anti-Bot Detection & Fallback](anti-bot-and-fallback.md) — Automatic retry with proxy escalation and fallback functions when anti-bot blocking is detected