Add optional Document Id to Documents

Elwig/Documents/Document.cshtml.cs

@@ -36,6 +36,7 @@ namespace Elwig.Documents {
         public string Header { get; set; }
         public string Footer { get; set; }
         public DateTime Date { get; set; }
+        public string? DocumentId { get; set; }
 
         private async Task<string> Render() {
             string name;