@roelfrenkema @theropologist

It's not built-in and not supported by official team working at Krita Foundation. You can find Stable Diffusion as a plugin made by someone externally to the team.
github.com/w4ffl35/krita_stabl

It requires downloading the model of the AI separately, and you'll need an account.

Stable Diffusion/LAION model is not Free/Libre and Open-Source compatible. It's a form of Open-Source with many rules on the top to prevent "AI Take Over" and unethical usage (how ironic).