doc: 96Boards Carbon: add Bluetooth setup docs

Add cross-referenced information on the 96b_carbon and
96b_carbon_nrf51 pages which disambiguates between the two "boards".
Also describe how to flash 96b_carbon_nrf51 with
samples/bluetooth/hci_spi and 96b_carbon with samples/bluetooth/ipsp
to support a Bluetooth HCI stack on 96Boards Carbon (the physical
board).

While we're here, make the documentation page for 96b_carbon match the
format in doc/templates/board.tmpl.

Signed-off-by: Marti Bolivar <marti.bolivar@linaro.org>