Ask HN: Tool for iterating through a codebase in a tutorial-like manner?

A few weeks ago a tool was featured here on HN.

In a web UI, multi-pane user interface, you could show documentation while at the same time iterating through specific parts of the code in another pane.

Does this sound familiar to anyone? I remember it having either `story`, `notebook` or `code` in its name, which makes it a pretty hard thing to search for. Thanks in advance!!!