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/Details.cshtml

@@ -16,10 +16,10 @@
 
 @if (!string.IsNullOrEmpty(Model.Invoice?.ResultFilePath))
 {
-    <button class="btn btn-primary mb-3 ms-2"
-            onclick="openPdfViewer(@Model.Invoice.Id)">
+    <a class="btn btn-primary mb-3 ms-2"
+       href="/Invoices/DocumentViewer?id=@Model.Invoice.Id">
         <i class="dx-icon-pdffile"></i> Ergebnis anzeigen
-    </button>
+    </a>
 }
 
 @if (Model.Invoice is null)