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/DocumentOperator.Domain.csproj

@@ -7,7 +7,6 @@
   </PropertyGroup>
 
   <ItemGroup>
-    <Folder Include="Common\Exceptions\" />
     <Folder Include="Common\Results\" />
     <Folder Include="Constants\" />
     <Folder Include="Models\Enums\" />