Update usage-next-13.mdx: Hint at loading.tsx files

[tordans]

The fact that layouts need to be handled differently in the app folder becomes very clear right from the start. However, the possible changes using the special loading files is a bit more hidden, which is why I suggest hinting at it in the migration docs.

[vercel]

@tordans is attempting to deploy a commit to the Blitz Team on Vercel.

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[siddhsuresh]

So while technically true, a loading.tsx file will wrap the entire page.tsx inside a suspense boundary, which users might not expect. Unless the dev is using invoke to get the data and render in page.tsx.

Otherwise keeping <Suspense> would be a good bet to suspend on the required parts of the UI especially when using useQuery and its siblings.

[tordans]

@siddhsuresh looking at a regular pages-page like https://github.com/blitz-js/blitz/blob/main/packages/generator/templates/page/__modelIdParam__.tsx#L84-L86 everything except for the one Link is part of the Suspense. Which is IMO 99% of what loading.tsx does, is it not? The only think i loos it this one link which only shows up after loading is done. I gain, however, a much simpler __modelIdParam__.tsx file with just one regular component.

The loading.tsx is still wrapped inside the nested layouts that apply to it, so it's still a Suspense/loading-animation in the context of the layout — AKA this part does not change.