[Docs] Perf tools

Fixes #1814.
@petehunt @joshduck
The images look blurry (taken on retina then shrank down to 50%...)
I think we should write a brief profiling tutorial or case study.

The reason for this is that this profiler is very bad at capturing absolute numbers especially since it only runs in dev. Instead, the profiler is designed to make it stupid simple to find the *relatively* expensive parts of your app and tell you *exactly* how to fix it.