refactor: enhance code documentation and remove unnecessary greeting outputs in tests

app/src/test/java/sudoku/TestSudokuLettres.java

@@ -9,7 +9,7 @@ public class TestSudokuLettres {
     @Test
     public void sudokuLettres() {
         System.out.println("TEST SUDOKU LETTRES : ");
-        System.out.println(new App().getGreeting());
+        
         // Create a new Sudoku
         Sudoku sudoku = new Sudoku(9);