Support deletion state for post summary child answers

[kylejrp]

Is your feature request related to a problem? Please describe.
In one part of the Stack Overflow codebase, we're adding .s-post-summary__deleted alongside .s-post-summary--answer to show that the parent is not deleted, but the child is. It happened to work out nicely:

https://codepen.io/kpollardso/pen/wvXPMeY

The documentation on post summary doesn't list this case on the page. There's also some other states that may come up in the future like ignored, watched, or pinned.

Describe the solution you'd like
I'd like this usage to be added to the documentation for post summary so we can document this as official usage. It would also be appreciated if you could review this usage and make any styling changes necessary for this new use case.

Describe alternatives you've considered
I was reaching for a generic Stacks utility that didn't exist, something like how Bootstrap does card backgrounds/color and card borders. It would be nice to be able to have something more top-level that we could use to designate some components as deleted. Something that would combine the font colors, border colors, and background colors all into one utility class so I can put .s-danger or something on a div and have it look like it's deleted.

Additional context
If you want a live example, this appears when you've saved one of your deleted answers into Saves on Stack Overflow, it's using our PostSummary view component thing.