docs: Add SearXNG to example configuration file

Update config.example.json to include SearXNG web search provider
configuration alongside existing Brave, DuckDuckGo, and Perplexity options.

This ensures users have a complete reference for all available search
providers when setting up their PicoClaw instance.

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

config/config.example.json

@@ -186,6 +186,11 @@
         "enabled": false,
         "api_key": "pplx-xxx",
         "max_results": 5
+      },
+      "searxng": {
+        "enabled": false,
+        "base_url": "http://localhost:8888",
+        "max_results": 5
       }
     },
     "cron": {