Improve docs to create your own widget

[batpad]

Part of #5

The docs to create a new widget at https://developmentseed.org/manywidgets/create-your-own-widget/ are quite confusing and make everything sound very complex.

This should be broken down:

• Start with a dead-simple JS widget - no build tooling, just a simple JS function that accepts render and a model and does something. Refer to upstream anywidget docs.
• Explain why of some of the manywidgets conventions: support theming, linking of widgets, etc.
• Explain how one can simply add those hooks and follow those styling conventions
• Explain why we use TypeScript and a simple build process to enforce types