This ticket is a part of #816 and will be used for creating new page titled "Custom Templates" 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/1RWwHFk5hPyFroURK1S7szNa3fKbagxUl5HG5oJqFtrw/edit?usp=sharing

Note: this wasn't included in the initial outline (template parts were but not custom templates), but this is a top-level property that is necessary.

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.