Refactor project structure and add new features

Restructured project files across all layers:
- Removed `Controllers` folder reference from `DocumentOperator.API.csproj`.
- Added folder structure to `DocumentOperator.Application`, `DocumentOperator.Domain`, and `DocumentOperator.Infrastructure` projects for better organization.

Introduced new API configurations and middleware:
- Added `SerilogConfiguration` and `SwaggerConfiguration` classes.
- Added `ExceptionHandlingMiddleware`, `RequestLoggingMiddleware`, and `TenantResolutionMiddleware`.

Implemented new document processing feature:
- Added `ProcessDocumentCommand`, `ProcessDocumentHandler`, and `ProcessDocumentValidator` classes in the application layer.

DocumentOperator.API/Middleware/ExceptionHandlingMiddleware.cs

@@ -0,0 +1,6 @@
+namespace DocumentOperator.API.Middleware
+{
+    public class ExceptionHandlingMiddleware
+    {
+    }
+}

DocumentOperator.API/Middleware/RequestLoggingMiddleware.cs

@@ -0,0 +1,6 @@
+namespace DocumentOperator.API.Middleware
+{
+    public class RequestLoggingMiddleware
+    {
+    }
+}

DocumentOperator.API/Middleware/TenantResolutionMiddleware.cs

@@ -0,0 +1,6 @@
+namespace DocumentOperator.API.Middleware
+{
+    public class TenantResolutionMiddleware
+    {
+    }
+}