fig-alt support for diagrams

Discussed in https://github.com/quarto-dev/quarto-cli/discussions/9000

^{Originally posted by felixvuo March 7, 2024}

Description

Here's a minimal document with a graphviz dot diagram. The fig-cap seems to work ok, but fig-alt does not give alternative text in the docx or html (and obviously not in pdf).

Is this something that should work? It would be really helpful - part of the motivation behind using quarto is to be able to produce accessible alternative formats for lectures, talks, etc.

---
title: "Minimum Working Example - Alt text"
author: "Me Myself"
format: 
  html:
    minimal: true
  beamer: 
    aspectratio: 169
  docx:
    toc: true
---

# First Things First

Here's a lovely diagram of something.

```{dot}
//| label: fig-tree
//| fig-alt: "A search tree rooted on node G"
//| fig-cap: "A game search tree"

graph G {
    node [label=""];    
    G -- a1;
    G -- a2;
    a1 -- a11;
    a1 -- a12;
    a1 -- a13;
    a2 -- a21;
    a2 -- a22;   
    G [label="G"];
}
```

That's all.
````</div>

Start with the minimal document and Graphviz diagram in the issue, then inspect its HTML, DOCX, and PDF output to compare how fig-cap and fig-alt are handled. Done means fig-alt produces appropriate alternative text in the supported output formats, including the expected behavior for PDF.