Updating the docs to Svelte 5

[malinowskip]

I managed to make the docs compile in Svelte 5 and I’m including the changes for reference here.

Non-obvious changes that made it work:

• After upgrading Prettier to v3, the format function expects a plugins array where prettier-plugin-svelte must be provided. Otherwise, it fails to resolve the parser and throws an error.
• For some reason, the docs only compile if the synchronous version of Prettier’s format is used.

Related: #1877 (comment)