docs: document IllegalStateException in DynamicTheme.initialize

Close #147

Author: javier-godoy

src/main/java/com/flowingcode/vaadin/addons/demo/DynamicTheme.java

@@ -146,6 +146,8 @@ public void initialize(AppShellSettings settings) {
    *
    * @param response the index HTML response to be modified
    * @throws UnsupportedOperationException if the runtime Vaadin version is older than 25
+   * @throws IllegalStateException if the {@link AppShellConfigurator} is configured with
+   * the legacy {@link Theme} annotation
    */
   public void initialize(IndexHtmlResponse response) {
     assertFeatureSupported();