docs: add sveltekit update and alt config

[MentalGear]

I struggled setting this up with sveltekit, and was only able to get it working with the added mergeConfig

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[mcous]

@timdeschryver I was perusing the open PRs list for Svelte stuff, and I do not think this PR is necessary anymore after #1389

[timdeschryver]

Thanks @mcous !