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

docs/app/@theme/services/code-loader.service.ts

@@ -0,0 +1,39 @@
+import { Injectable } from '@angular/core';
+import { HttpClient } from '@angular/common/http';
+import { Observable } from 'rxjs';
+import { publishReplay ,  refCount } from 'rxjs/operators';
+
+@Injectable()
+export class NgxCodeLoaderService {
+
+  /**
+   * Contains cached files by url.
+   * */
+  private cache: Map<string, Observable<string>> = new Map();
+
+  constructor(private http: HttpClient) {
+  }
+
+  load(path: string): Observable<string> {
+    const url = this.buildFilePath(path);
+    const cached = this.cache.get(url);
+
+    return cached ? cached : this.buildRequest(url);
+  }
+
+  private buildFilePath(path: string): string {
+    return `assets/examples/${path}`;
+  }
+
+  private buildRequest(url): Observable<string> {
+    const request = this.http.get(url, { responseType: 'text' })
+      .pipe(
+        publishReplay(1),
+        refCount(),
+      );
+
+    this.cache.set(url, request);
+
+    return request;
+  }
+}