Refactor: Rename DocumentOperator to DocumentService

Updated project and solution files to rename and restructure
`DocumentOperator` to `DocumentService`. Modified project
references in `DocumentService.API.csproj`, `DocumentService.Application.csproj`,
and `DocumentService.Infrastructure.csproj` to reflect the new
naming convention. Updated `DocumentService.sln` to remove
old project references, add new ones, and adjust configuration
and nested project mappings accordingly. This change aligns
the project structure with the new naming convention.

DocumentService.Infrastructure/Configuration/DocumentServiceSettings.cs

@@ -0,0 +1,11 @@
+namespace DocumentService.Infrastructure.Configuration;
+
+public class DocumentServiceSettings
+{
+    public const string SectionName = "DocumentServiceSettings";
+
+    public string TempFolderPath { get; set; } = string.Empty;
+    public int TempFileRetentionHours { get; set; }
+    public int MaxPdfSizeMB { get; set; }
+    public bool EnableDetailedLogging { get; set; }
+}