Refactor contact info handling in PDF signing

Replaced the `ContactInfo` property with separate `Email` and `Phone` fields in `PdfSignOptionsDto` and `PdfSignatureCheckItemDto` for better granularity. Updated the `PdfDigitalSignatureService` to handle the new fields, introducing `BuildContactInfo` and `ParseContactInfo` methods for constructing and parsing contact information.

Modified the `PdfSignatureController` to reflect these changes, updating request models and the `SignPdfAsync` method. Updated the `README.md` to document the new structure. These changes improve the clarity and structure of contact information handling.

EnvelopeGenerator.Application/Common/Dto/PdfSignOptionsDto.cs

@@ -8,7 +8,8 @@ public sealed record PdfSignOptionsDto
     public string? Name { get; set; }
     public string? Reason { get; set; }
     public string? Location { get; set; }
-    public string? ContactInfo { get; set; }
+    public string? Email { get; set; }
+    public string? Phone { get; set; }
     public string? ApplicationName { get; set; }
     public PdfSignatureCertificationLevel? CertificationLevel { get; set; }
 }