Add domain exceptions and update project structure

Implemented a structured exception-handling mechanism in the
domain layer with the addition of `DomainException`,
`DomainValidationException`, `NotFoundException`, and
`PdfProcessingException` classes. These exceptions provide
specific error handling for domain logic and integrate with
centralized middleware.

Updated `ROADMAP.md` to mark Step 2.1 (Domain Exceptions) as
completed and Step 2.2 (Value Objects) as the next task.
Added timeline entries to reflect progress.

Cleaned up `DocumentOperator.Domain.csproj` by removing
unused folder inclusions, indicating a project structure
reorganization.

DocumentOperator.Domain/Common/Exceptions/PdfProcessingException.cs

@@ -0,0 +1,34 @@
+namespace DocumentOperator.Domain.Common.Exceptions;
+
+/// <summary>
+/// Exception thrown when PDF processing operations fail.
+/// Maps to HTTP 500 Internal Server Error or 422 Unprocessable Entity in the API layer.
+/// </summary>
+public class PdfProcessingException : DomainException
+{
+    public string Operation { get; }
+
+    public PdfProcessingException(string operation, string message)
+        : base($"PDF processing failed during '{operation}': {message}", "PDF_PROCESSING_ERROR")
+    {
+        Operation = operation;
+    }
+
+    public PdfProcessingException(string operation, string message, Exception innerException)
+        : base($"PDF processing failed during '{operation}': {message}", "PDF_PROCESSING_ERROR", innerException)
+    {
+        Operation = operation;
+    }
+
+    public PdfProcessingException(string message)
+        : base(message, "PDF_PROCESSING_ERROR")
+    {
+        Operation = "Unknown";
+    }
+
+    public PdfProcessingException(string message, Exception innerException)
+        : base(message, "PDF_PROCESSING_ERROR", innerException)
+    {
+        Operation = "Unknown";
+    }
+}