doc: Add instructions for 5G NR gNB frequency configuration
- link to the file: https://gitlab.eurecom.fr/oai/openairinterface5g/-/blob/doc-frequency-config/doc/gNB_frequency_setup.md?ref_type=heads
@luis_pereira87 @francescomani @schmidtr, can you please check what I wrote?
Edited by Jaroslava Fiedlerova