Add API docs and example for device inclusion, especially the S2 flow

[AlCalzone]

[raman325]

is there a particular use case that triggered this or was this just a general thought?

[AlCalzone]

Came up in Discord today. Someone tried figuring out how to include an S2 device via the server.
It's not exactly obvious I have to say.

[raman325]

it definitely not obvious - we've always assumed that this would be embedded elsewhere and wouldn't be used by end users.

At some point I explored providing some sort of introspection in the API but lost track of it after a while. We can definitely provide some documentation though