docs: add prerequisits about output.ssg

[targeral]

Summary

Add prerequisits content about output.ssg documentation.

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Checklist

• I have added changeset via pnpm run change.
• I have updated the documentation.
• I have added tests to cover my changes.

[changeset-bot]

⚠️ No Changeset found

Latest commit: a557ab6

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.

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