Remove Nitrogen and nRF52840 DK from list of supported HW in RTD

We will not support Nitrogen and nRF52840 DK in Oniro. The tasks is about:

removing it from the RTD section (https://docs.oniroproject.org/en/latest/oniro/hardware-support/index.html)
checking (grep) if the board are mentioned in other places in docs and discuss with Adam about removing

Edited Sep 21, 2022 by Grzegorz Gwóźdź

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