Convert our basic UI pages (terminal, editor, etc) to React pages

Let's leverage the [Docusaurus custom pages](https://docusaurus.io/docs/en/custom-pages#adding-other-custom-pages) technique to host our basic UI. We need to convert things to be simple pages within our "docs" site and add top-level menu links so people can jump around.

This way we can mix documentation and live content into one thing.