Add documentation about data stream notifications

[paul121]

Currently there is no documentation about data stream notifications. Not sure if this would be better in the user guide or development section. I think there are aspects that pertain to both.

[mstenta]

That would be great. I think the user guide makes sense for the first step. I could see a development section geared towards more advanced sensor usage/integrations, but that feels separate enough that it doesn't need to happen at the same time.

I could almost see a whole new page for Data Streams in the user guide - perhaps linked to from https://farmos.org/guide/assets/#sensors.