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

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

@@ -0,0 +1,29 @@
+import { Injectable } from '@angular/core';
+
+export interface NgxTocElement {
+  fragment: string;
+  element: any;
+  y: number;
+  setInView(val: boolean);
+}
+
+@Injectable()
+export class NgxTocStateService {
+  state: NgxTocElement[] = [];
+
+  add(el: NgxTocElement) {
+    this.state.push(el);
+  }
+
+  remove(el: NgxTocElement) {
+    this.state = this.state.filter(e => e !== el);
+  }
+
+  list(): NgxTocElement[] {
+    return this.state;
+  }
+
+  clear() {
+    this.state = [];
+  }
+}