Add DevExpress Document Viewer page and dependencies

Updated `_ViewImports.cshtml` to include DevExpress and DevExtreme tag helpers. Created `DocumentViewerDevExpress.cshtml` for displaying a DevExpress Document Viewer with necessary CDN links. Implemented the corresponding code-behind in `DocumentViewerDevExpress.cshtml.cs` to handle model binding and report key generation.

DXApp.TemplateKitProject/Pages/Invoices/DocumentViewerDevExpress.cshtml.cs

@@ -0,0 +1,16 @@
+using Microsoft.AspNetCore.Mvc;
+using Microsoft.AspNetCore.Mvc.RazorPages;
+
+namespace DXApp.TemplateKitProject.Pages.Invoices;
+
+public class DocumentViewerDevExpressModel : PageModel
+{
+    [BindProperty(SupportsGet = true)]
+    public int Id { get; set; }
+
+    public string ReportKey => $"invoice-{Id}";
+
+    public void OnGet()
+    {
+    }
+}