Update localization design doc to use `PoseWithCovarianceStamped`
PoseWithCovarianceStamped
is the de-facto standard output type of localization modalities.
It has the semantics of being the localization of the base_link
frame with respect to whatever frame is specified in the header::frame_id
field.
The high level localization design document should be updated to reflect this de-facto standard.