docs: add SearXNG providers guide for unblocking Google behind Gluetun VPN

SearXNG queries all ~50 upstream engines from one Gluetun VPN exit IP, so
rate-sensitive engines (Google, DuckDuckGo) answer with reCAPTCHA/429 and
SearXNG suspends the offending engine for hours-to-days, showing up as
"unavailable". Add docs/Dankclaw/SearXNG Providers.md covering: the
suspension model, the two failure modes (CAPTCHA vs selector drift), engine
config (settings.yml engines: list — duplicate google, timeout,
retry_on_http_error, JSON feed), request tuning (outgoing block, UA rotation,
suspended_times — correcting the old rate_limit/useragents blog myths),
routing/proxies (per-engine proxies, dedicated 2nd Gluetun, dedicated IP), and
verification (/json, /config, check.searx.space). Crosslink into SearXNG.md
and index.md.

docs/index.md

@@ -51,6 +51,7 @@ covers both workloads).
 ## Dankclaw Services
 
 - [[SearXNG]] — privacy metasearch (VPN-routed)
+- [[SearXNG Providers]] — unblocking Google & other upstream engines behind the VPN
 - [[Gluetun]] — the VPN tunnel container
 - [[Valkey]] — SearXNG cache and limiter store
 - [[ComfyUI]] — node-based AI image generation (CPU)