revealjs logo option's alt text support is undocumented

[cwickham]

revealjs logo option's alt text support is undocumented

The logo option for format: revealjs accepts an object form with path
and alt fields, and the alt value renders as the image's alt
attribute. The reference and guide pages show only the string form and never
mention alt text.

This YAML gives the logo an accessible name:

format:
  revealjs:
    logo:
      path: logo.png
      alt: "Company logo"

The docs pages I checked show only the plain string form and say nothing
about alt:

• https://quarto.org/docs/reference/formats/presentations/revealjs.html
  — the logo entry reads "Logo image (placed in bottom right corner of
  slides)", with no example and no mention of an object form.
• https://quarto.org/docs/presentations/revealjs/index.html (section
  "Footer & Logo") — the only example is logo: logo.png.