allow to serialize the "build" section

src/config.rs

@@ -352,6 +352,11 @@ impl Serialize for Config {
         let book_config = Value::try_from(&self.book).expect("should always be serializable");
         table.insert("book", book_config);
 
+        if self.build != BuildConfig::default() {
+            let build_config = Value::try_from(&self.build).expect("should always be serializable");
+            table.insert("build", build_config);
+        }
+
         if self.rust != RustConfig::default() {
             let rust_config = Value::try_from(&self.rust).expect("should always be serializable");
             table.insert("rust", rust_config);

tests/init.rs

@@ -91,6 +91,12 @@ fn run_mdbook_init_with_custom_book_and_src_locations() {
             file
         );
     }
+
+    let contents = fs::read_to_string(temp.path().join("book.toml")).unwrap();
+    assert_eq!(
+        contents,
+        "[book]\nauthors = []\nlanguage = \"en\"\nmultilingual = false\nsrc = \"in\"\n\n[build]\nbuild-dir = \"out\"\ncreate-missing = true\nuse-default-preprocessors = true\n"
+    );
 }
 
 #[test]