Improve documentation and comments for TensorFlow/Keras import examples

[NA-V10]

This PR enhances the clarity of the KerasMNISTImportExample by improving inline comments and adding explanations for each major step:

• Loading MNIST test dataset
• Importing TensorFlow/Keras .h5 model into DL4J
• Running evaluation and interpreting results

No functional changes were made to the example.
The purpose of this update is to help new users understand how to correctly import and run pretrained Keras models using DL4J.