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.
