Centralise documentation for the output widget

[pbugnion]

Currently, documentation for the output widget is scattered across:

• async widgets
• widget list

With PR #1934 , we need to extend the docs for the output widget to expose its use as a decorator.

Given the output widget's power, we think it makes sense to bring docs on the output widget to the top level. I think it would be useful to structure the notebook as:

• introduction (what's currently in the widget list)
• displaying rich output (e.g., with an example from matplotlib?)
• async widgets
• using the @capture decorator, including as a way to debug widget callbacks.

Happy to tackle this today / this weekend, unless someone else is particularly keen to.

---

Jason deserves the credit for the idea behind this issue.

[jasongrout]

Definitely +1. I started an output widget notebook a while ago, but I'm not sure I pushed it anywhere.

[jasongrout]

Here is the output notebook I started working on a while ago. Feel free to use it as a start, or feel free to discard it and do what you think is best: https://gist.github.com/anonymous/eb176705a103517179b4f57b6201df1d

[jasongrout]

#1936 also has some interesting material that could be incorporated in an output notebook.

[jasongrout]

Fixed by #2020. Thanks again!