Documentation: gpio: Move gpiod_* consumer documentation to driver-api

Move gpio/consumer.txt to driver-api/gpio/consumer.rst and make sure it
builds cleanly as ReST.

Signed-off-by: Jonathan Neuschäfer <j.neuschaefer@gmx.net>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>