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

docs/app/@theme/services/dialog-state.service.ts

@@ -0,0 +1,23 @@
+/**
+ * @license
+ * Copyright Akveo. All Rights Reserved.
+ * Licensed under the MIT License. See License.txt in the project root for license information.
+ */
+
+import { Injectable } from '@angular/core';
+import { Observable, ReplaySubject } from 'rxjs';
+import { share } from 'rxjs/operators';
+
+@Injectable()
+export class DialogStateService {
+
+  protected dialogState$ = new ReplaySubject();
+
+  changeDialogState(state: string) {
+    this.dialogState$.next({state});
+  }
+
+  onChangeDialogState(): Observable<any> {
+    return this.dialogState$.pipe(share());
+  }
+}