Added documentation for LED-ring with CF21-BL #1956

Job	Run time
read_targets_from_file / read_targets_from_file	3s
basic_build (cf2)	3m 25s
basic_build (bolt)	3m 24s
basic_build (tag)	3m 34s
basic_build (flapper)	3m 29s
basic_build (cf21bl)	3m 31s
kbuild-targets (allyesconfig)	3m 28s
apps (examples/app_hello_world)	2m 48s
kbuild-targets (allnoconfig)	3m 22s
features (bosch.conf)	3m 29s
apps (examples/app_hello_world-cpp)	2m 55s
features (loco_tdma.conf)	3m 29s
apps (examples/app_hello_file_tree)	2m 59s
kbuild-targets (randconfig)	3m 18s
features (loco_tdoa2.conf)	3m 35s
apps (examples/app_peer_to_peer)	2m 58s
apps (examples/app_p2p_DTR)	2m 55s
features (loco_tdoa3.conf)	3m 34s
apps (examples/app_stm_gap8_cpx)	2m 57s
features (loco_tdoa3_all.conf)	3m 31s
apps (examples/demos/app_push_demo)	2m 49s
features (bigquad.conf)	3m 28s
apps (examples/demos/swarm_demo)	2m 58s
features (app_api.conf)	3m 28s
apps (examples/demos/app_wall_following_demo)	3m 2s
	1h 18m 29s

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