This ticket is a part of #816 and will be used for creating new page titled "Template Parts" under Chapter 3: Global Settings and Styles

I will draft the content for the new page in a Google Doc: https://docs.google.com/document/d/1iR8A5ZQygHRRHKkjfvLpmWmumjEstCA7LMYPbqX5UHQ/edit?usp=sharing

Chapter 4: Templates Required

Before publishing this document, we also need to publish the Templates chapter. This doc is a reference to adding templates via theme.json, but the primary documentation will be the Templates chapter.