docs: headers for snippet prop section

[Rich-Harris]

This is an alternative to #15172, which feels a little duplicative to me. Splitting the section up into three subsections makes it easier to scan for the example you're looking for while keeping the linear flow. Closes #14020, Closes #15172

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[svelte-docs-bot]

Preview: https://svelte-dev-git-preview-svelte-15745-svelte.vercel.app/

[github-actions]

Playground

pnpm add https://pkg.pr.new/svelte@15745

[dummdidumm]

We might need a forth sub heading then since the text in the last subsection explains more than the implicit children

[Rich-Harris]

done