Docs: provide instructions how to use Dev Proxy with Postman

[waldekmastykarz]

[waldekmastykarz]

It seems like Dev Proxy just works with Postman. @garrytrinder let's have another look at the scenario that made us think we need docs for this.

[waldekmastykarz]

Not planning to work this as of now

[waldekmastykarz]

Reopening based on customer feedback