Ensure images and interactives are accessible

[siwelwerd]

I believe currently very few of our images have descriptions which will produce alt text, which is necessary to make our books screen reader accessible. We should make sure every image has a <shortdescription>, and probably most of them would benefit from a <description> as well. See Subsection 4.40.3: Advice for Writing Image Descriptions for a longer explanation.

Note that <shortdescription>s should be limited to 125 characters and must be plain text. A <description> could use things like <m> tags.

Edit: All <interactive>s should have a <description> as well. I updated the Style guide to indicate Doenet as the preferred interactive tool (as pending a bug fix they should be keyboard-navigable), or if a 3D interactive is required a Sageplot with a detailed <description>.

[siwelwerd]

@StevenClontz Does Doenet have any accessibility features at all? I can't find anything on their website describing how a visusally impaired user might interact with them. Compared to say https://www.desmos.com/accessibility .

Edit: They should be navigable by keyboard, though their documentation is lacking.