doc: boards: Move nRF5x DK board doc from the wiki to git

This includes:
* Moving the Nordic nRF5x Segger J-Link page (temporarily placed inside
one of the board folders)
* Moving the nrf52_pca10040 doc
* Moving the nrf51_pca10028 doc
* Moving the nrf52840_pca10056 doc

Change-Id: I051eb51cee8166ae6472eb696ffeb0625a0424c7
Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>