Add tutorial for debugging with proxy to docs

[heitortsergent]

The tutorial from @tom-miseur on k6 Load Testing Debugging Using a Web Proxy is extremely useful, and I think it would be good for us to have that as part of, and maintained, in the docs.

We should also include a section that talks about how it's focused on debugging tests locally, and how you could use it to debug cloud tests and any drawbacks from it.

Related community forum post.