Enhance PDF viewing experience with new document viewer

- Replaced button in `Details.cshtml` with a link to a new document viewer page.
- Created `DocumentViewer.cshtml` to display PDFs using an iframe, with a back button and a link to an experimental DevExpress viewer.
- Introduced `DocumentViewerModel` in `DocumentViewer.cshtml.cs` for handling invoice ID and PDF URL.
- Updated `OnGetAsync` in `ViewPdf.cshtml.cs` to set `Content-Disposition` to inline for better PDF rendering in the browser.

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

@@ -0,0 +1,17 @@
+using Microsoft.AspNetCore.Mvc;
+using Microsoft.AspNetCore.Mvc.RazorPages;
+
+namespace DXApp.TemplateKitProject.Pages.Invoices;
+
+public class DocumentViewerModel : PageModel
+{
+    [BindProperty(SupportsGet = true)]
+    public int Id { get; set; }
+
+    public string PdfUrl => $"/Invoices/ViewPdf?id={Id}";
+
+    public void OnGet()
+    {
+        // Keine serverseitige Logik nötig für die erste Version.
+    }
+}