DOC: Update style guide to refer to the template notebook in pymc-examples repo

[drbenvincent]

Issue with current documentation:

The pymc-examples repo now has a template notebook: https://github.com/pymc-devs/pymc-examples/blob/main/template_notebook.ipynb

The issue is that there is not yet any reference to this in the style guide.

Idea or request for content:

The style guide should be updated to refer to this template notebook.