Improve the WebSockets documentation once the experimental extension is considered stable

[immavalls]

Based on https://community.k6.io/t/max-number-of-websocket-connection/5663/13, I think we could document this a bit better for our users.

There is a good example in https://github.com/grafana/k6/releases/tag/v0.40.0, and it's worth documenting why built the extension. And what we recommend in terms of usage.

I myself missed this when going over the docs. I only saw the examples in https://k6.io/docs/using-k6/protocols/websockets/#load-testing-websockets-with-k6 and https://k6.io/docs/examples/websockets/ and missed https://k6.io/docs/javascript-api/k6-experimental/websockets/.