docs(config): update documentation for settings structure (#7352)

docs/sandbox.md

@@ -55,7 +55,9 @@ gemini -p "run the test suite"
 
 # Configure in settings.json
 {
-  "sandbox": "docker"
+  "tools": {
+    "sandbox": "docker"
+  }
 }
 ```
 
@@ -65,7 +67,7 @@ gemini -p "run the test suite"
 
 1. **Command flag**: `-s` or `--sandbox`
 2. **Environment variable**: `GEMINI_SANDBOX=true|docker|podman|sandbox-exec`
-3. **Settings file**: `"sandbox": true` in `settings.json`
+3. **Settings file**: `"sandbox": true` in the `tools` object of your `settings.json` file (e.g., `{"tools": {"sandbox": true}}`).
 
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