[cleanup] Updated wiki pages to the new xar format.
XWIKI-10296: The default value of a date field should be empty or today, not the date when the class has been created
- Using a magic date to mark inside an application's template that a default value is not actually set and that the emptyIsToday mechanism should still be applied when creating new entries.
- Used a custom displayer for Date fields that displays an empty value in the AWM class editor or the entry's edit/create form when the magic value is set to the template so that the user is never exposed to this technical detail.