Document explicitly how to connect Fins

[lucamrgs]

It is not yet evident from the documentation how and where to implement and connect fins.

The sequence for connecting a fin is roughly:

1. SOARCA is run with enable_fins to true
2. SOARCA interfaces an MQTT broker container to which a Fin process should connect
3. any Fin process connects to the MQTT container, and registers as fin to SOARCA according to protocol (evtl. using our libraries)
4. Playbooks which use the fin need to specify a soarca-fin agent. The "name" property of such fin agent should be consistent with the name of the Fin registered via protocol
5. Now all is set to execute playbooks with SOARCA fins

It would be nice to have this process described explicitly in the documentation