Add section organization to 11_training module: Add DEVELOPMENT section header

- Insert ## 🔧 DEVELOPMENT header before first test function
- Organizes module according to educational structure guidelines
- Maintains all existing functionality and test execution
- Improves readability and navigation for educational use

modules/source/11_training/training_dev.py

@@ -186,6 +186,8 @@ class MeanSquaredError:
 
 # %% [markdown]
 """
+## 🔧 DEVELOPMENT
+
 ### 🧪 Unit Test: MSE Loss
 
 Let's test our MSE loss implementation with known values.