fix(docs-infra): improve skip-to-main-content method to focus <main> element instead of h1

Update the skip-to-main-content behavior to focus the <main> element when present, with a fallback to the first heading for legacy layouts without a main landmark.

adev/src/app/app.component.ts

@@ -82,6 +82,14 @@ export class AppComponent {
   }
 
   protected focusFirstHeading(): void {
+    const main = this.document.querySelector<HTMLElement>('main');
+    if (main) {
+      main.setAttribute('tabindex', '-1');
+      main.focus();
+      return;
+    }
+
+    // Fallback: focus the first h1 (legacy support for pages without main)
     const h1 = this.document.querySelector<HTMLHeadingElement>('h1:not(docs-top-level-banner h1)');
     h1?.focus();
   }