Documents: Fix letterhead and other problems

Elwig/Documents/Letterhead.cs

@@ -1,10 +1,21 @@
 using Elwig.Models.Entities;
+using iText.Kernel.Pdf;
+using iText.Kernel.Pdf.Canvas;
+using iText.Layout;
 
 namespace Elwig.Documents {
     public class Letterhead : BusinessDocument {
         public Letterhead(Member m) :
             base($"Briefkopf {m.FullName}", m, true) {
-            Aside = null;
+        }
+
+        protected override void RenderBody(iText.Layout.Document doc, PdfDocument pdf) {
+            // do not render anything except this
+            var page = pdf.AddNewPage();
+            var pageSize = page.GetPageSize();
+            var pdfCanvas = new PdfCanvas(page.NewContentStreamBefore(), page.GetResources(), pdf);
+            using var canvas = new Canvas(pdfCanvas, pageSize);
+            RenderAddress(canvas, pageSize);
         }
     }
 }