lab: update Arm lab docs

The one node and two node Arm testbeds were improperly named. They were
configured with a new hostname in the past but the docs were not updated
accordingly, so fix the docs.

Create a new server type for Arm servers and servers related to Arm
testbeds to improve doc organization.

Change-Id: I205b1e2f958dd1113e1e9f207acffe4355b267bc
Signed-off-by: Juraj Linkeš <juraj.linkes@pantheon.tech>