Allow styled figure notes using the new 1.4 crossref figure syntax

From discussion https://github.com/quarto-dev/quarto-cli/discussions/6103

New 1.4 crossref syntax allow to add text under a figure as shown: https://github.com/quarto-dev/quarto-cli/discussions/6103#discussioncomment-7296412

---
title: "Reproducible Quarto Document"
format: pdf
fig-cap-location: top
---

::: {#fig-1}

```{r}
#| echo: false
plot(1:10)
```

Source: My source

My figure caption.

:::

See @fig-1.

Suggestion by @danielvartan is to have a way to consider the content below the figure that is not a caption as some figures notes properly identified.

• In LaTeX this could be using a command like \legend{} that could be redefined the the user,
• In HTML, it could be using a new div that would allow targeted styling

Use case are from academic and others conventions where Figures' Note are required.

FWIW, this is a long standing feature request in knitr

• https://github.com/yihui/knitr/issues/2022

Current workaround:

The 1.4 syntax allow any paragraph to be added below the figure content, and separated from the Caption. It could be styled as needed as shown in this exemple: https://github.com/quarto-dev/quarto-cli/discussions/6103#discussioncomment-7503264

Related issues

This FR could be seen as related to

• https://github.com/quarto-dev/quarto-cli/issues/645

Put it could happen that Figure caption are located at the top and Notes below the figures, while having different style. Example:

Start with the linked discussion 6103 and related issue 645, then compare the documented 1.4 syntax and current workaround. The issue names no repository files or tests; done would mean figure notes can be distinguished from captions and styled appropriately in both LaTeX and HTML.