Update routes to include '/example' prefix in .razor files

Updated the `@page` directives in `DocumentViewer.razor`,
`ReportDesigner.razor`, `ReportViewer.razor`, and `TestViewer.razor`
to prepend the `/example` prefix to their respective routes.

This change modifies the URL paths for accessing these components
to better organize or namespace the routes under the `/example`
prefix. No functional or structural changes were made to the
components themselves.

EnvelopeGenerator.ReceiverUI/Pages/Example/DocumentViewer.razor

@@ -1,4 +1,4 @@
-@page "/documentviewer"
+@page "/example/documentviewer"
 
 <DxDocumentViewer ReportName="LargeDatasetReport" CssClass="dx-blazor-reporting-container" Height="@null" Width="@null">
     <DxDocumentViewerTabPanelSettings Width="340" />

EnvelopeGenerator.ReceiverUI/Pages/Example/ReportDesigner.razor

@@ -1,4 +1,4 @@
-@page "/sender"
+@page "/example/sender"
 @using DevExpress.DataAccess.Json;
 @using EnvelopeGenerator.ReceiverUI.Services;
 

EnvelopeGenerator.ReceiverUI/Pages/Example/ReportViewer.razor

@@ -1,5 +1,5 @@
-@page "/receiver/{EnvelopeKey}"
-@page "/report-viewer/{EnvelopeKey}"
+@page "/example/receiver/{EnvelopeKey}"
+@page "/example/report-viewer/{EnvelopeKey}"
 @using System.Drawing
 @using DevExpress.Blazor
 @using DevExpress.Drawing

EnvelopeGenerator.ReceiverUI/Pages/Example/TestViewer.razor

@@ -1,4 +1,4 @@
-@page "/test"
+@page "/example/test"
 @using System.Drawing
 @using DevExpress.Drawing
 @using DevExpress.Utils