[docs] Add info about promo variant of `<rh-card>`

[marionnegp]

Description

Need to add some info about the promo variant of <rh-card> on the Style and Guideline subpages. Currently the only promo info we have on ux.redhat.com appears under demos.

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Additional context

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[marionnegp]

@markcaron @hellogreg, this can be kicked to Cubone if you want to focus on some of the docs bugs cropping up. I can make it a fast follow after Clefairy's release.

[bennypowers]

let's kick this. anyways we should do some deep design work on tokens -> elements -> theming -> patterns