doc: update doc links from preview to main

[leonardsimonse]

Docs still have quite a few references to links on the Svelte 5 preview site. Some references have also moved, now that the new docs site is up. I've changed all remaining links to their new destination.

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[benmccann]

Thanks! We should definitely fix these. It looks like there's a number of merge conflicts unfortunately. Would you mind resolving the conflicts?

[dummdidumm]

The links have since been updated, therefore closing - thank you!