Add custom exception classes and Extensions folder

Introduce four custom exceptions (BurnAnnotationException, CreateReportException, ExportDocumentException, MergeDocumentException) under EnvelopeGenerator.ServiceHost.Exceptions for improved error handling. Update the project file to include the new Extensions folder.

EnvelopeGenerator.ServiceHost/Exceptions/MergeDocumentException.cs

@@ -0,0 +1,12 @@
+namespace EnvelopeGenerator.ServiceHost.Exceptions;
+
+public class MergeDocumentException : ApplicationException
+{
+    public MergeDocumentException(string message) : base(message)
+    {
+    }
+
+    public MergeDocumentException(string message, Exception innerException) : base(message, innerException)
+    {
+    }
+}