<rh-card> Make the API more flexible

[markcaron]

IMO, we should make the API to card more flexible, as in not requiring a header or footer, so that it can be used as a building block with other components, like <rh-stat>, etc.

It would also open up the possibility for it to handle the need for an Icon Panel, which is basically a card with an icon at the top. I believe this would simplify things greatly.

[bennypowers]

currently card doesn't require header or footer and uses SlotController to hide those containers if the associated slotted content isn't present. (in the future we can maybe use :has() for this

[markcaron]

Do we extend the API of card to also cover "Sticky Card"?

XD mockup:
https://xd.adobe.com/view/d20a1c17-375c-4c7f-6827-5fe2f8c2d92f-a43b/screen/ab37d0fd-94c5-423a-ac5e-fe69951d492c/

[markcaron]

Also discussed in PFE:
patternfly/patternfly-elements#1614

[bennypowers]

My initial impression is that the sticky card from that issue should either be:
a) its own specialized component (so as not to add excess unused code to the regular card), or
b) a modeless <rh-dialog>, perhaps even wrapped in a 3rd component

<rh-sticky>
  <rh-dialog open>
    <h4 slot=header>Card Title</h4>
    <h5 slot=description>Heading, xs</h5>
    <p>Lorem ipsum dolor sit amet, consectetur adipiscing elit,
       sed do eiusmod tempor incididunt ut labore et dolore magna aliqua.</p> 
    <rh-cta><a href="/yadda/yadda">Call to action</a></rh-cta>
  </rh-dialog>
</rh-sticky>

From MDN:

The <dialog> HTML element represents a dialog box or other interactive component, such as a dismissible alert, inspector, or subwindow.

This matches up with "dismissable alert" nicely.