docs: remove svelte.dev section from README.md

[SymphonySimper]

Content in the svelte.dev section was outdated. So just updated the links and wording.

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[Conduitry]

The update from the sites repo to the svelte.dev repo is good, but we should continue to point to the svelte repo for content, because everything in https://github.com/sveltejs/svelte.dev/tree/main/apps/svelte.dev/content/docs/svelte is copied/generated from the svelte repo.

That said, I wouldn't be opposed to just removing this whole subsection in the readme. We already have links throughout the site that point to the appropriate repo and file that you should be editing to change that current page.

[SymphonySimper]

Yes, that makes sense. I also think we can remove this section, as users can already edit the appropriate docs using the Edit this page on GitHub link.