app: theme-ui being used in the editor but not documented

[aarongarciah]

Subject of the issue

Cards templates use theme-ui but it's not described anywhere: nor the app, nor the docs. I assume most of the users won't be familiar with this.

I think it's worth it to add a note explaining this and a link to theme-ui docs. Maybe good to add which version of them-ui is being used too.

edit: I see now that this is mentioned 👉 https://microlink.io/docs/cards/getting-started/editor, maybe a little bit hidden?