Add time series notebook

[amadeuspzs]

Summary

What/Why

This PR adds a notebook version of the time series walkthrough extracted from PDF.

What to review

• notebooks/4.Time_Series_Forecast.ipynb

Checklists

This pull/merge request meets the following requirements:

• [ ] code runs
• [X] developments are ethical
• [X] developments are secure (no data or notebook output)
• [X] you have made proportionate checks that the code works correctly
• [X] minimum usable documentation written in the docs folder

This notebook requires training data to run; it has been tested using #21, but at the time of this PR the data used does not render in the UI due to separate integration issues. Once those are resolved, I do not expect any of the code in this notebook to change.

[amadeuspzs]

This PR is superseded by #21