doc/examples/Makefile: Install bts-mgr cfg files used by systemd services
doc/examples/Makefile: Install bts-mgr cfg files used by systemd services

Change-Id: Iaf362da2a9ebd6829391637252812c2be93e9c60

osmocom/osmo-bts,osmocom/osmo-bts,osmocom/osmo-bts