feat(button): 2nd-gen migration plan

[5t3ph]

Description

Provides the migration plan for Button, per washing machine guidelines.

NOTE: Awaiting official a11y plan (SWC-1874) before this can be finalized and merged.

Related issue(s)

• SWC-1875

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Reviewer's checklist

1. Ensure all expected content is included in the migration plan
2. Help answer any outstanding questions, especially blockers
3. Help validate/provide feedback for scope, API, and changes/additions

[changeset-bot]

⚠️ No Changeset found

Latest commit: 9941ddf

Merging this PR will not cause a version bump for any packages. If these changes should not result in a new version, you're good to go. If these changes should result in a version bump, you need to add a changeset.

This PR includes no changesets

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[github-actions]

📚 Branch Preview Links

• Documentation Site (first-gen)
• Storybook (first-gen)
• Storybook (second-gen)

🔍 First Generation Visual Regression Test Results

When a visual regression test fails (or has previously failed while working on this branch), its results can be found in the following URLs:

• Spectrum | Light | Medium | LTR
• Spectrum | Dark | Large | RTL
• Express | Light | Medium | LTR
• Express | Dark | Large | RTL
• Spectrum-two | Light | Medium | LTR
• Spectrum-two | Dark | Large | RTL
• High Contrast Mode | Medium | LTR

Deployed to Azure Blob Storage: pr-6176

If the changes are expected, update the current_golden_images_cache hash in the circleci config to accept the new images. Instructions are included in that file.
If the changes are unexpected, you can investigate the cause of the differences and update the code accordingly.