[Accessibility] fig-alt does not work for mermaid

Currently, fig-alt does not work for mermaid diagram. However, there seems a workaround. Mermaid has their own accessibility options.

@cscheid -- We may want to bind fig-cap to accTitle and fig-alt to accDescr. Other additional accessibility fixes that I am not mentioning here need to be taken care of on mermaid side.

Reprex

---
title: "Untitled"
format: html
---

# Mermaid Accessibility

## Inaccessible Version

```{mermaid}
%%| fig-alt: testing

flowchart LR
  A[Hard edge] --> B(Round edge)
  B --> C{Decision}
  C --> D[Result one]
  C --> E[Result two]
```


## Accessible Version

```{mermaid}
graph LR
      accTitle: Big Decisions
      accDescr: Bob's Burgers process for making big decisions
      A[Identify Big Descision] --> B{Make Big Decision}
      B --> D[Be done]
```