These are new features and are available in the
working version of the User's Guide (scroll to bottom file).
Shortcut:
Put your files in the user settings folder (you can easily go there by clicking the "Open Preferences Location" button in the lower left corner of the Preferences window).
For EPUB2:
user-template2.xhtml file (+ optionally
user-template2.css with styles).
For EPUB3:
user-template3.xhtml file (+ optionally
user-template3.css with styles).