docs: move %sveltekit.version% to the correct section

[teemingc]

Follow up to #12132

The %sveltekit.version% string is only available in app.html, not error.html so it should be moved under it in the docs.

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[svelte-docs-bot]

Preview: https://svelte-dev-git-preview-kit-14593-svelte.vercel.app/

[Rich-Harris]

ah whoops. though i have to wonder if version should be available in error.html too. separate question though

[teemingc]

ah whoops. though i have to wonder if version should be available in error.html too. separate question though

Probably same for the other interpolated strings. There was an issue asking to have access to assets so that they can link the same stylesheet.