[Feature]: Use Jupytext for cleaner notebook diffs

### Problem Statement

Notebook diffs are often noisy because they include saved outputs.

### Proposed Solution

I suggest using [Jupytext](https://github.com/jupytext/jupytext) to pair notebooks with text-based source files (eg .py files), making changes easier to review and merge.

We could trial it on a couple of notebooks first and decide how to handle syncing, outputs, and widgets.

### Alternatives Considered

_No response_

### Additional Context

_No response_

### Checklist

- [x] I have read the [contributing guidelines](/bloomberg/causal-ts/blob/main/CONTRIBUTING.md)
- [x] I have verified this does not duplicate an existing feature request

Start by selecting the couple of notebooks for the proposed Jupytext trial and review how their saved outputs and widgets are currently handled. Read Jupytext's pairing and syncing guidance, then define a documented workflow whose completion criteria cover text-based diffs, synchronization, outputs, and widgets.

Written by the indexing model from the issue text.