media: Documentation: Rename meta format files

Rename meta format files, using "metafmt" prefix instead of "pixfmt-meta".
These are metadata formats, not pixel formats.

Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Cc: Bingbu Cao <bingbu.cao@intel.com>
Cc: Dafna Hirschfeld <dafna@fastmail.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>