Address 402 scenario feedback

[rolznz]

• I think this is too much detail and talks about the proxy internals. Is that really helpful for somebody using the sandbox? I would maybe even remove the "protected URL" input and just use some default URL.
• and as that 402 stuff is for sure not widely understood let's add the fow diagram for the different protocols?
• Add a note the user can open the browser dev tools and do the request again to review the raw http responses