added alt-text to tutorial-deep-learning-mnist

[cooperrc]

I added alt-text to the figures in the deep-learning MNIST tutorial.

One question (we briefly discussed in the alt-text sprint started in issue #96):

• How do we want to provide information from matplotlib figures?

I added a "caption" in italics below the code-cell e.g.

```{code-cell} ipython3
plt.plot(np.arange(10), np.arange(10)**2)
```
_Shown above, the `arange` function is used to create an x-axis from 0-9 and a y-axis of x-squared._

[rossbar]

Thanks everyone for taking a look/sharing thoughts about how to handle generated images. LGTM!