Improve docs to create your own widget

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