Add alt text for figure images

[geealbers]

To increase the accessibility of the book, alt descriptions should be added to all figure images in the book. Follow Getty Publications' Alt Text Guidelines when crafting the descriptions.

Work off of the revisions branch, and be sure to note the Node and Quire CLI versions the project uses.

Please submit a chapter's worth as a pull request to start. After which you can proceed and your own pace and submit a PR whenever you feel it's ready.