AvatarStack: document variant and shape props in docs and Storybook

[liuliu-dev]

The variant and shape props on AvatarStack were not documented or set up in the Storybook Playground. This was discovered while investigating github/core-ux#2196, where the "Used by" sidebar section could use variant="stack" to match the Rails AvatarStack spacing.

Changelog

Before:

Screen.Recording.2026-04-20.at.3.01.16.PM.mov

After:

Screen.Recording.2026-04-20.at.3.01.00.PM.mov

Rollout strategy

• Patch release
• Minor release
• Major release; if selected, include a written rollout or migration plan
• None; if selected, include a brief description as to why

Testing & Reviewing

Merge checklist

• Added/updated tests
• Added/updated documentation
• Added/updated previews (Storybook)
• Changes are SSR compatible
• Tested in Chrome
• Tested in Firefox
• Tested in Safari
• Tested in Edge
• (GitHub staff only) Integration tests pass at github/github-ui (Learn more about how to run integration tests)

[changeset-bot]

⚠️ No Changeset found

Latest commit: 21ff32f

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[liuliu-dev]

skipping changeset and integration tests since only docs and storybook are changed

[francinelucca]

:ship-it: