docs/index: rauc extract should not be used on the target
docs/index: rauc extract should not be used on the target

From time to time we see users trying to build automation around rauc extract
for use on the target, which doesn't fit well with the idea that a bundle
describes the full intended system state.

Signed-off-by: Jan Luebbe <3a82f1ac4ad59607a20f180f1c67ac8ba31ad0d5@pengutronix.de>

ejoerns/rauc,rauc/rauc,rauc/rauc,jluebbe/rauc,jluebbe/rauc,ejoerns/rauc,ejoerns/rauc,jluebbe/rauc,rauc/rauc