boards: arm: nrf: change board docs to link to Nordic Infocenter

Link to Nordic Semiconductor Infocenter, instead of DocLib
in the documentation for nRF-based Development Kits.

Change Nordic Semi to Nordic Semiconductor in the
DK figure captions.

Signed-off-by: Ioannis Glaropoulos <Ioannis.Glaropoulos@nordicsemi.no>