Migrate the docs to Svelte 5

[baseballyama]

And at this time, can we move the docs directory under monorepo?
I'm not sure about best practice for this but personally, monorepo is used in many cases.

[ota-meshi]

I think using monorepo is a good idea!
I think we can probably remove some ESM<->CJS compatibility hacks for building doc site.

[dummdidumm]

Heads up: We're likely going to create a monorepo for svelte.dev which should become the home for all documentation around Svelte including first-party tooling, which includes the eslint plugin. So in the long term it would be nice to have the documentation in there.

[baseballyama]

After release v3, I will migrate the docs to omnisite.