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.Tests/TestData/Pdfs/encryptedPdf.pdf

@@ -0,0 +1,30 @@
+%PDF-1.4
+1 0 obj
+<< /Type /Catalog /Pages 2 0 R >>
+endobj
+
+2 0 obj
+<< /Type /Pages /Kids [3 0 R] /Count 1 >>
+endobj
+
+3 0 obj
+<< /Type /Page /Parent 2 0 R /MediaBox [0 0 612 792] >>
+endobj
+
+4 0 obj
+<< /Filter /Standard /V 2 /R 3 /Length 128 /P -3904 /O (OwnerHash12345678901234567890123456) /U (UserHash12345678901234567890123456) >>
+endobj
+
+xref
+0 5
+0000000000 65535 f 
+0000000009 00000 n 
+0000000058 00000 n 
+0000000115 00000 n 
+0000000206 00000 n 
+
+trailer
+<< /Size 5 /Root 1 0 R /Encrypt 4 0 R >>
+startxref
+370
+%%EOF