Add docs for widget container height option

See https://github.com/matrix-org/matrix-react-sdk/pull/12893

config.sample.json

@@ -22,6 +22,7 @@
         "https://scalar-staging.vector.im/api",
         "https://scalar-staging.riot.im/scalar/api"
     ],
+    "default_widget_height": "280px",
     "default_country_code": "GB",
     "show_labs_settings": false,
     "features": {},

docs/config.md

@@ -452,6 +452,11 @@ If you would like to use Scalar, the integration manager maintained by Element,
 }
 ```
 
+For widgets in general (from an integration manager or not) there is also:
+ * `default_widget_container_height`
+
+This controls the height that the top widget panel initially appears as and is the height in pixels, default 280.
+
 ## Administrative options
 
 If you would like to include a custom message when someone is reporting an event, set the following Markdown-capable field: