[Docs] remove nitrogen and nrf from used machines

Related to docs#154 (closed)

Right now I only removed those boards from /docs/hardware-support/boards/index.rst and docs/build-flavours/zephyr-flavour.rst

Signed-off-by: Grzegorz Gwóźdź grzegorz.gwozdz@huawei.com

docs/build-flavours/zephyr-flavour.rst

@@ -22,10 +22,8 @@ Supported machines (default in **bold**):
 
 * **qemu-x86**
 * qemu-cortex-m3
-* 96b-nitrogen (96Boards Nitrogen)
 * 96b-avenger96 (96Boards Avenger96)
 * arduino-nano-33-ble (Arduino Nano 33 BLE and Arduino Nano 33 BLE Sense)
-* nrf52840dk-nrf52840 (Nordic Semiconductor nRF 52840 Development Kit)
 
 Build steps example: