Page Templates are html documents used when displaying website pages.
Page Templates are stored in the Page Templates site content. To edit Page Templates, go to the admin site and click the Design icon, and click Page Templates from the flyout.
The html-Import tool (under the Tools icon) lets you upload a Page Template with it's assets (images, styles, js) and perform transformations that modify the template to help a designer add and remote elements.
See Design > html-import for more details
Use the Page Template's Add-on field to implement Mustache customizations to the template.
See Design > Mustache Templating for more details