Add support for `alt` (and maybe `title`) for `placeholder-image` in listings

[dragonstyle]

Discussed in #6987

^{Originally posted by maelle September 25, 2023}

Description

I have two closely related questions about images for website listings

• How could one add the title for an image, that is to say the text available on hover? I was thinking it'd be a good place to add credit (to the image creator).
• When using an image placeholder, can one add an alternative text?

Thank you. 🙏