boards: renesas: Rework rcar_salvator_xs doc

Rework renesas rcar_salvator_xs doc to meet what has
been done for the documentation of other rcar boards.
Also take advantage of these new boards documentation
features:
- zephyr:board
- zephyr:board-supported-hw
- zephyr:board-supported-runners

Signed-off-by: Aymeric Aillet <aymeric.aillet@iot.bzh>