sensor: sensor_axis_align: clarify defgroup

Rename and clarify the @defgroup SENSOR_AXIS_ALIGN to better reflect its
purpose. Replace the vague "DT Options" title with a more descriptive
name: "Sensor axis alignment DT options".

This improves documentation clarity for developers configuring sensor
orientation via Device Tree.

Signed-off-by: Gaetan Perrot <gaetan.perrot@spacecubics.com>