chapter 3: display of digits from MNIST dataset.

```python
def plot_digits(instances, images_per_row=10, **options):
size = 28
images_per_row = min(len(instances), images_per_row)
images = [instance.reshape(size,size) for instance in instances]
n_rows = (len(instances) - 1) // images_per_row + 1
row_images = []
n_empty = n_rows * images_per_row - len(instances)
images.append(np.zeros((size, size * n_empty)))
for row in range(n_rows):
rimages = images[row * images_per_row : (row + 1) * images_per_row]
row_images.append(np.concatenate(rimages, axis=1))
image = np.concatenate(row_images, axis=0)
plt.imshow(image, cmap = mpl.cm.binary, **options)
plt.axis("off")
```

I can't understand the code, It would be a great help if you could simplify it.

Start with the plot_digits function shown in the issue and the surrounding Chapter 3 material. Clarify whether the goal is a simpler explanation or a rewritten example, then make the MNIST grid-building steps understandable to a newcomer; check the successor repositories first because this repository is deprecated.

Written by the indexing model from the issue text.