feat: add post summary micro variant

[dancormier]

resolves #966

See https://deploy-preview-967--stacks.netlify.app/product/components/post-summary/#micro

Questions

• Should we truncate titles? If so, how many lines? (This could be handled in core as needed. Curious if we should prescribe truncation though. There's implications when it comes to vertically aligning elements within the post summary.)
• What's the recommended title font size? (Profile currently uses body2, sidebar currently uses body1)
• Should items include borders? (I'm assuming they can be added as needed by consumers but I'm not sure if we want to unify this.)
• Should we include support for timestamps? Or any other optional data, for that matter? (Profile currently includes timestamp)

TODOs

• [ ] Update documentation

Screenshot

Second draft

Below are screenshots with a couple variations (unanswered/short title/no date/no menu, longer title/short date/longest title/long date). There's more that could exist in these post summaries.

First draft

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[paintedbicycle]

• I think body1 seems fine
• Yes, the component should handle truncation
• Do we need the Q and A icon back? And the Article and Collection icons?

[paintedbicycle]

Looking at the profile page I think we need the icons, otherwise everything would look exactly the same. This could then also be used in places like the sidebar and in search like below

[dancormier]

I'm closing this PR since it will have many, many conflicts once https://github.com/StackExchange/Stacks/pull/1091 is merged. We shall revisit post-PPCP refactor.