boards/nucleo: group nucleo boards in documentation

boards/nucleo-common/include/board_common.h

boards/nucleo-f030/include/board.h

boards/nucleo-f070/include/board.h

boards/nucleo-f072/include/board.h

boards/nucleo-f091/include/board.h

boards/nucleo-f103/include/board.h

boards/nucleo-f302/include/board.h

boards/nucleo-f303/include/board.h

boards/nucleo-f334/include/board.h

boards/nucleo-f401/include/board.h

boards/nucleo-f410/include/board.h

boards/nucleo-f411/include/board.h

boards/nucleo-f446/include/board.h

boards/nucleo-l053/include/board.h

boards/nucleo-l073/include/board.h

boards/nucleo-l1/include/board.h

boards/nucleo-l476/include/board.h

boards/nucleo144-common/include/board_common.h

boards/nucleo144-f207/include/board.h

boards/nucleo144-f303/include/board.h