[NEW, CHANGE] Better image descriptions

[Rob Beezer]

We've greatly improved how an author can supply written descriptions of images,
largely as part of our commitment to accessibility.

New element, child of "image", "shortdescription": this is "alt text", short
(~125 characters), zero markup ("straight ASCII"), except WeBWorK "var" element
is allowed.

Recycled element, child of "image", "description": much richer, you *must*
structure this with a mix of "p" and "tabular".

Backward-compatibility: The pre-processor will see your current unstructured
"description" and move it to a new "shortdescription" (and has been doing this
for a few weeks already). So no rush. You can manually migrate your
unstructured "description" to a "shortdescription" by changing the element name,
then, possibly later, add a new "description" that is more expressive.

Presentation of "shortdescription": as before, into HTML "alt" attribute,
transcriber note for braille.

Presentation of "description": HTML "details" disclosure widget integrated into
screen readers, transcriber note for braille with full rendering (not yet,
simple task to do). Examples at:

Figure 9.8
https://pretextbook.org/examples/sample-article/html/section-graphics.html#figure-pgfplots-data-description

Figure 9.17
https://pretextbook.org/examples/sample-article/html/section-graphics.html#figure-sage-exosagevec1

Authoring: if you've tried writing these descriptions, you know how hard it is.
And David F argues convincingly that AI will never do the job right. However,
David F organized a seminar on this topic, given by Michael Cantino, and Oscar
has distilled the essence of that into our documentation.

Subsection 4.38.3: Advice for Writing Image Descriptions
https://pretextbook.org/doc/guide/html/topic-accessibility.html#subsec-img-desc-advice

Thanks to Alex for the implementation for HTML, and David F, Michael Cantino,
and Oscar for the author education. I think yours truly may have helped a
little bit here and there.

With some use in the wild, I think something similar for "interactive", "video",
"audio" could be a routine addition.

Rob