Updates for blackboxing

ref #176

- resized images to fit side-by-side
- added some headings and collapsable sections
- added link in js-debugging to blackboxing plus some text and screenshots of the feature below the async-call-stacks part

https://rawgit.com/jaredwilli/devtools-docs/blackboxing/docs/blackboxing.html

@Garbee can you review?