doc: services: shell: add reference to west rtt

Adds a reference to west rtt as alternative to using putty. Also provides
a command that prints supported rtt runners for the specific build.

This issue is encountered now with the nRF54L1, for example, because
the nrfutil runner does not support RTT at this time.

Signed-off-by: Helmut Lord <kellyhlord@gmail.com>