Add Summary2 support for missing components

The following component types are currently missing Summary2 support:
- `Accordion`
- `AccordionGroup`
- `Cards`

I believe all of these can be handled somewhat similarly to `Group` and `Tabs`, but cards might need its own presentation style.

When this is done, I think we can make the `renderSummary2()` function abstract, making it a required implementation detail for new layout components.

Start by locating the Summary2 and renderSummary2 entry points, then compare the existing Group and Tabs implementations. Check how Accordion, AccordionGroup, and Cards are represented and determine the presentation expected for Cards; done means all three support Summary2 and renderSummary2 can be made abstract for new layout components.

Written by the indexing model from the issue text.