docs: demonstrate sv migrate over prior commands

[benmccann]

New package available at https://www.npmjs.com/package/sv

[changeset-bot[bot]]

🦋 Changeset detected

Latest commit: 22292dc77e06d25c35766d3143a100639d121031

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 2 packages

Name	Type
svelte-migrate	Patch
@sveltejs/package	Patch

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