Add accessibility lints to CI

I'm not sure if there is an easy way to add this to Zola projects, but it would be nice to have basic accessibility checks in CI.
Stuff like "every `img` element has an `alt` attribute" should be enough for the start.

If Zola has anything built-in we could use that, otherwise maybe it's possible to do the linting on the generated HTML?

Start by locating the repository's CI configuration and the Zola build entry point. Check whether Zola provides an accessibility-linting option; otherwise inspect the generated HTML workflow and determine how to report missing alt attributes. Done means CI runs the checks and flags an image without an alt attribute.

Written by the indexing model from the issue text.