Add captions and alt text to figures

[troyraen]

In the notebooks listed in #184, add captions and alt text to all figures. I don't think the notebooks use any static images, but if they do we should add captions and alt text to those as well.

Captions: We could add a markdown cell with the caption right below the cell generating the figure. I don't know of another way to do this.

Alt text: This is the html alt attribute that screen readers can use. The text doesn’t display visually unless the image itself can’t be displayed for some reason. From @bsipocz:

IMO we should rely on mpl’s alt-text generation as we cannot really guarantee that what’s generated is in fact aligns with what we mean to generate. Now, this is easier said than done, I’m not sure how good these alt-text generators are.

Sounds like we need to investigate a little more.

[troyraen]

The captions should be easy. If the alt text proves difficult then let's just write down a question or two to ask the consultants. That's enough for this issue.

[troyraen]

The consulting firm is already looking at notebooks, so making changes to these three is not strictly necessary. We should at least investigate the alt text so know how easy/hard that will be and can ask questions. The consulting firm may or may not be able to give us python-specific advice but can certainly help answer more general things. For example, if we find the alt text to be difficult we can ask whether they can point us to any resources and how important it is to pursue.