docs: remove previews

[wolfy1339]

Followup to octokit/endpoint.js#382 (comment)
See also octokit/endpoint.js#388
Requires octokit/types.ts#491

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Behavior

Before the change?

• Previews were documented

After the change?

• Previews are no longer documented

Other information

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Additional info

Pull request checklist

• Tests for the changes have been added (for bug fixes / features)
• Docs have been reviewed and added / updated if needed (for bug fixes / features)
• Added the appropriate label for the given change

Does this introduce a breaking change?

Please see our docs on breaking changes to help!

• Yes (Please add the Type: Breaking change label)
• No

If Yes, what's the impact:

• Removal of previews

Pull request type

Please add the corresponding label for change this PR introduces:

• Bugfix: Type: Bug
• Feature/model/API additions: Type: Feature
• Updates to docs or samples: Type: Documentation
• Dependencies/code cleanup: Type: Maintenance

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[github-actions]

🎉 This PR is included in version 6.0.0 🎉

The release is available on:

• GitHub release
• npm package (@latest dist-tag)

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