Update example scripts for new test.k6.io site

[na--]

The old 90s test.loadimpact.com site has been updated, and now lives at https://test.k6.io, so we have to update our example scripts (including the README)

[ppcano]

I think we should remove the samples folder and move the focus to https://k6.io/docs/examples

We ported a few of these examples on the samples folder to the examples page of the documentation. Not sure if we should port more of them.

[na--]

I personally like to have the samples in the k6 repo. Maybe we can prune some of them, and improve others, but I don't see a huge issue of having a few simple scripts. We can always add a README.md in the samples/ folder that points to the big list in https://k6.io/docs/examples

Having them in the k6 repo allows simple k6 run github.com/loadimpact/k6/samples/whatever.js demos, is personally useful for me when developing k6, and would also probably be useful for #625.