samples: bme280: provide generic arduino I2C and SPI support

Adjust the documentation and devicetree overlays so the sample can be
built for any board with an Arduino I2C and SPI pinout, defaulting I2C
and SPI to y to make it easier to switch between the two without
requiring a pristine build.

The user has to choose an appropriate overlay or have a sensor built
in to the board.

Use the newly introduced DEVICE_DT_GET_ANY() in main.c to ask for a
bosch,bme280 without worrying over the details or exposing
DT_DRV_COMPAT-based functionality that is really meant for drivers.

Remove the no-longer-needed board specific overlay for nRF52840 DK;
this is covered by the generic Arduino overlays now.

Fix the datasheet link while we're here.

Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>