Create section dedicated to the CP UI

[leevigraham]

As a plugin developer I want to make sure my plugins match Crafts UI as closely as possible.

To make this a better UX it would be great if the docs included a UI style guide section which described markup and classes. Currently I'm reverse engineering components by looking at the twig source templates.

I fear that with craft3 and the plugin store everyone will start using custom UI's and we'll end up with inconsistent experiences.

The main customisation I've needed so far is nested fieldsets. Here's how I handled it in a plugin I'm currently working on. I had to customise the labels of the "sub" fields and create a container.