docs: driver-api: gpio: Fix filename mismatch

The filenames were changed a while ago, but board.rst, consumer.rst and
intro.rst still refer to the old names. Fix those references to match the
Actual names and avoid possible confusion.

Signed-off-by: Tom Schwindl <schwindl@posteo.de>
Signed-off-by: Bartosz Golaszewski <brgl@bgdev.pl>