Document debugger better (breakpoints, etc.)

[jamesonquinn]

The current documentation is three short paragraphs and a gif. An example of how to set a breakpoint (including how to import/use/include the code where the breakpoint will go) would be great in avoiding false starts.

[ChrisRackauckas]

May be premature to do before JuliaDebug/Gallium.jl#187

[pfitzseb]

It might not even be worth to write anything up before the 0.6 compatible version of Gallium is released, because I'd kinda expect things to change once that happens.
But yeah, the Juno docs are still somewhat lacking in many places... I hope I'll find some time to write some docs during GSoC :)