feat: added demo button to header

feat: changed buttons in landing's main section

docs/app/@core/data/service/header-menu.service.ts

@@ -15,6 +15,10 @@ export class HeaderMenuService {
       title: 'Docs',
       link: '/docs',
     },
+    {
+      title: 'Demo',
+      url: 'https://www.akveo.com/ngx-admin?utm_campaign=ngx_admin%20-%20demo%20-%20ngx_admin%20docs&utm_source=ngx_admin&utm_medium=referral&utm_content=landing_main_section',
+    },
   ];
 
   getHeaderMenu(): Observable<NbMenuItem[]> {