Document theming for custom components

[anicholls]

🚀 Feature Proposal

The helpful useTheme() hook and styled function are not documented here: https://github.com/Workday/canvas-kit/blob/master/modules/react/common/lib/theming/README.md

Motivation

We should add documentation for building your own custom components using the custom theme so people know they can use it in their own components!

[RayRedGoose]

It's still an issue, and requires update documentation on theming.

It good to mention styled function, because it helps to automatically switch direction-sensitive styles for RTL and LTR directions).
useTheme is used in one example (breakpoints) but no overview and information about other functionality.

Also, storybook theming page is pretty empty, so we need to add all this info to storybook too.

[jaclynjessup]

#923

[myvuuu]

Closing this out since we'll be revisiting and documenting theming as part of the v10 work.