Make .addSection fully declarative

demo/8-header-footer.ts

@@ -3,20 +3,22 @@
 import * as fs from "fs";
 import { Document, Footer, Header, Packer, Paragraph } from "../build";
 
-const doc = new Document();
-
-doc.addSection({
-    headers: {
-        default: new Header({
-            children: [new Paragraph("Header text")],
-        }),
-    },
-    footers: {
-        default: new Footer({
-            children: [new Paragraph("Footer text")],
-        }),
-    },
-    children: [new Paragraph("Hello World")],
+const doc = new Document({
+    sections: [
+        {
+            headers: {
+                default: new Header({
+                    children: [new Paragraph("Header text")],
+                }),
+            },
+            footers: {
+                default: new Footer({
+                    children: [new Paragraph("Footer text")],
+                }),
+            },
+            children: [new Paragraph("Hello World")],
+        },
+    ],
 });
 
 Packer.toBuffer(doc).then((buffer) => {