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/DomainException.cs

@@ -0,0 +1,25 @@
+namespace DocumentOperator.Domain.Common.Exceptions;
+
+/// <summary>
+/// Base exception for all domain-related exceptions.
+/// Caught by the Exception Handling Middleware in the API layer.
+/// </summary>
+public abstract class DomainException : Exception
+{
+    /// <summary>
+    /// Error code for categorization and logging.
+    /// </summary>
+    public string ErrorCode { get; }
+
+    protected DomainException(string message, string errorCode) 
+        : base(message)
+    {
+        ErrorCode = errorCode;
+    }
+
+    protected DomainException(string message, string errorCode, Exception innerException) 
+        : base(message, innerException)
+    {
+        ErrorCode = errorCode;
+    }
+}