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/ReportDesigner.razor

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