Add a documentation for localization initialization

[IshitaTakeshi]

The localization module (NDT) requires the initial pose. If GNSS is available the pose can be initialized, but if not NDT cannot obtain a reliable initial pose. In the case that the initial pose cannot be obtained, there's a function in autoware to manually provide an initial pose from rviz but there's no in the documentation that explains the usage.
@YamatoAndo