docs: disable IPyParallel & load dependencies in reverse order

[agoose77]

I've noticed intermittent failures in the "Try It" demo; sometimes we get a symbol definition error. However, it transpires that re-running the import fixes this. I added an explicit import for awkward-cpp that appears to fix the issue.

This PR also disables IPyParallel, which is a nuisance.

[jpivarski]

It's better for the "Try it!" experience to be stable and robust. It's probably fine for it to be a little slower; maybe most users (right now) understand that running Python in a browser is a different, less performant platform than regular CPython.

Also, I'm surprised that a parallel-processing extension would have any kind of speedup in a browser, which is async but not parallel. It's still the case that the installations use the await keyword.

[agoose77]

I think IPyParallel is just a default extension that gets installed with IPython(?) I've not dug into where it comes from in great depth. In any case, we weren't using it, but it was causing a noisy popup that the server extension wasn't loaded!