add self-closing-tags info

[Rich-Harris]

makes info about the migration script easier to find, along with a more detailed explanation about why self-closing tags are silly

https://svelte-dev-git-preview-svelte-14758-svelte.vercel.app/docs/svelte/compiler-warnings#element_invalid_self_closing_tag

[changeset-bot]

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Latest commit: 6cc0d0f

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Playground

pnpm add https://pkg.pr.new/svelte@14758

[Rich-Harris]

preview: https://svelte-dev-git-preview-svelte-14758-svelte.vercel.app/

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