Document alt text for the dashboard navbar logo

A format: dashboard navbar logo renders as a link wrapping an <img>, and unless the author supplies alt text it ships with alt="" — leaving the link with no accessible name (axe link-name, serious; WCAG 2.2 SC 2.4.4 Link Purpose (In Context)). Quarto has two working ways to set that alt text, but neither is documented anywhere:

format:
  dashboard:
    logo:
      path: images/penguins.png
      alt: "Palmer Penguins"

format:
  dashboard:
    logo: images/penguins.png
    logo-alt: "Palmer Penguins"

Both verified on Quarto 1.10.12: the alt lands on the rendered logo and gives the logo link its accessible name. The object form has been schema-valid since 1.8 (it accepts the full logo-light-dark-specifier, including per-light/dark objects); logo-alt has been honored by the dashboard template since dashboards debuted in 1.4 and is still explicitly supported (format-dashboard.ts#L126-L140). Alt can also come from a _brand.yml named logo resource (logo: images: <name>: {path, alt}).

Two places need it:

1. Dashboard layout guide, Navigation section — currently says only "You can also include a logo and one or more nav-buttons", and the YAML example uses the bare string form. Adding alt to that example (plus a sentence saying why) would make the accessible form the copy-paste default.

2. Dashboard format reference — the logo entry's description ("Logo image(s) (placed on the left side of the navigation bar)") doesn't mention alt or the object form, and logo-alt is absent entirely. This page is auto-generated from the schema, and logo-alt is missing from document-dashboard.yml (it's only defined for website navbars/sidebars), so fixing the reference likely requires a schema change rather than a quarto-web edit.

An AI assistant helped investigate, grounded in a local clone of this repo (per CONTRIBUTING.md).

Related: #14660 tracks what the default should be when no alt is provided; #13131 requests logo-href for dashboards. This issue is only about documenting the alt options that already work.

Start with the Navigation section in quarto.org/docs/dashboards/layout.html and the dashboard format reference, then inspect document-dashboard.yml and the linked format-dashboard.ts implementation. Update the navigation example and descriptions to cover accessible logo alt text and the object form, and ensure the generated reference includes the supported dashboard option.