chore: setup docs previews

[Rich-Harris]

this will, in theory, result in preview deployments for PRs that change the docs

[changeset-bot]

⚠️ No Changeset found

Latest commit: 40ec8c0

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[pkg-pr-new]

pnpm add https://pkg.pr.new/svelte@14128

commit: 40ec8c0

[Rich-Harris]

preview: https://svelte-dev-git-preview-svelte-14128-svelte.vercel.app/

this is an automated message