customMenuItem: menuPath config not not being used

[criticalmash]

The spec for navigation.customMenuItem includes a use of the menuPath variable as a shorthand for data: {'menu-path': 'menu/path'}. But the variable isn't being used.

Update customMenuItem to use this variable.

The documentation also needs to be updated to show the proper way to specify a top-level nav item. These items should have a format of ./[NAME]. The dot indicates it's a top-level item.