docs: add config file

[benmccann]

See sveltejs/svelte#12410. I didn't rename the files here yet to avoid breaking the currently deployed site. We can do that after retiring kit.svelte.dev

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[PuruVJ]

Quick question: It is automatically generated, so why are we adding redundant config?

[benmccann]

More of the description is in the Svelte PR, but the idea is that we want to publish docs for things like the below and we would either need to rename those files and directories to include numbers or use a config or support both. It's probably easier just to support one method and folks seemed to prefer the config route.

https://github.com/sveltejs/vite-plugin-svelte/tree/main/docs
https://github.com/sveltejs/eslint-plugin-svelte/tree/main/docs