sensors: Redefine SENSOR_CHAN_DISTANCE to be in meters

Most of sensor channels defined by Zephyr use the main, unscaled
SI unit. We also have SENSOR_CHAN_ALTITUDE which is defined in
meters. So, it only makes sense to define distance in meters too

The only driver supporting SENSOR_CHAN_DISTANCE as of now is
vl53l0x.c, which was updated accordingly. Also, update doc links
in the driver based on the review comment.

Fixes: #5693

Signed-off-by: Paul Sokolovsky <paul.sokolovsky@linaro.org>