Renamed and restructured project references across the solution
to replace `DocumentOperator` projects with `DocumentService`
projects. Updated all `.csproj` files to reflect the new project
names and references.
Modified the solution file (`DocumentOperator.sln`) to remove
`DocumentOperator` projects and add corresponding `DocumentService`
projects, including `DocumentService.API`, `DocumentService.Application`,
`DocumentService.Infrastructure`, `DocumentService.Domain`, and
`DocumentService.Tests`.
This change aligns the project structure with the new naming
convention or organizational standards.
