feat(demo): add landing page with docs (#1951)

docs/app/@theme/services/iframe-communicator.service.ts

@@ -0,0 +1,25 @@
+import { Inject, Injectable } from '@angular/core';
+import { Observable,  fromEvent as observableFromEvent } from 'rxjs';
+import { filter,  map } from 'rxjs/operators';
+import { NB_WINDOW } from '@nebular/theme';
+
+@Injectable()
+export class NgxIframeCommunicatorService {
+
+  constructor(@Inject(NB_WINDOW) private window) {
+  }
+
+  public send(payload: any, target: Window = this.window.parent) {
+    if (target !== this.window) {
+      target.postMessage(payload, '*');
+    }
+  }
+
+  public receive(id: string): Observable<any> {
+    return observableFromEvent(this.window, 'message')
+      .pipe(
+        filter((msg: any) => msg.data && msg.data.id === id),
+        map((msg: any) => msg.data),
+      );
+  }
+}