docs(svelte-testing-library): simplify examples with link to STL repository

[mcous]

I just got through porting the Svelte examples to v5, and I went ahead and did it in the STL repository itself so I could ensure that examples would be automatically tested (which caught a bug, which was cool).

In order to minimize the maintenance burden of the Svelte docs, this PR removes most examples from the testing-library.com docs in favor of linking to the STL repository for more complex examples.

I kept the basic example in place, because I think it's helpful to have something here, and updated it to the same Svelte 5 example that now also lives in the STL repo.

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[mcous]

Hey @timdeschryver! Yup this one is still good. Will tag you next time I need to put something up