Overhaul the dev server

Make the dev server more useful, by adding documentation to help people understand DCAR, and features for making day-to-day development easier. Possible improvements include:

- Documenting the targets DCAR can render for (dotcom, live apps, etc.).
- Documenting the types of content DCAR can render (articles, fronts, etc.).
- Providing lots of examples of content for development and testing purposes. This may include different kinds of pages, different formats of article, and different features (e.g. elements in articles).
- Providing tools to better understand articles in particular, including features for surfacing information about format and elements.
- Some form of better integration with storybook.
- More React and TypeScript, and less JavaScript.