Add alt text to figures on authoring pages

[cwickham]

Fixes 12 image-alt violations (critical, WCAG 2.0 A, SC 1.1.1) that the axe site audit found on docs/authoring/ pages. Each was an image with no alt attribute.

Changes

File	Fix
article-layout.qmd	fig-alt on four knitr cells (six figures)
cross-references.qmd	fig-alt on the identifier-only subtable screenshot
_cross-references-listings.qmd	fig-alt on the fig-plot cell
contents.qmd	fig-alt on the contents shortcode screenshot
cross-references-divs.qmd	fig-alt on the image used as a table
penguins-qmd.qmd	fig-alt on fig-size-scatter, which notebook-embed.qmd embeds
cell-alignment.html	alt on the placeholder image in the cell alignment demo

Three cells in article-layout.qmd use echo: fenced, so the new fig-alt option is visible in the rendered code. This also shows readers how to use the option.

Not fixed: placeholder.qmd

The two images on placeholder.qmd keep no alt text. They come from the {{< placeholder >}} shortcode, which returns pandoc.Image({}, result) with an empty alt and accepts no alt argument. An author can work around this with ![alt text]({{< placeholder 600 400 >}}), but this page documents the shortcode itself. A change to its examples changes what the page teaches. A fix belongs upstream in quarto-cli.

Freeze notes

article-layout, cross-references, penguins-qmd, and tables are frozen. Each was rendered from the repository root with R 4.5.2, which is the R version that renv.lock records. Every package used matches renv.lock exactly (0 version mismatches across all 165 locked packages).

Two results in this PR need an explanation:

1. Figure PNGs change. These are not version drift. A render in the exact locked environment reproduces the bytes in this PR, so the copies on main were stale relative to renv.lock. This PR brings them into line.
2. leaflet-binding-2.2.3 and htmltools-fill-0.5.9 are new. renv.lock pins leaflet 2.2.3 and htmltools 0.5.9, so these paths are correct. The freeze on main still pointed at 2.2.1 and 0.5.8.1. The older directories stay because docs/presentations/revealjs/demo still references them.

The penguins-qmd freeze holds two results, html.json and ipynb.json. On main these two had different hashes, and the stale ipynb.json was the reason the embedded figure lost its alt text. Both now share the same hash.

Verification

No image lacks an alt attribute on any edited page:

grep -o '<img [^>]*>' _site/docs/authoring/<page>.html | grep -v 'alt='

article-layout, contents, cross-references, cross-references-divs, notebook-embed, penguins-qmd, and tables all return nothing. placeholder still returns its two images.

[github-actions]

📝 Preview Deployment

🔍 Full site preview: https://deploy-preview-2138.quarto.org

🔄 Modified Documents

• docs/authoring/article-layout.html
• docs/authoring/contents.html
• docs/authoring/cross-references-divs.html
• docs/authoring/cross-references.html
• docs/authoring/penguins-qmd.html

[github-actions]

📝 Preview Deployment

🔍 Full site preview: https://deploy-preview-2138.quarto.org

🔄 Modified Documents

• docs/authoring/article-layout.html
• docs/authoring/contents.html
• docs/authoring/cross-references-divs.html
• docs/authoring/cross-references.html
• docs/authoring/penguins-qmd.html