Define JSON/AVRO data structure
In #368 (moved) we spelled out the O4 requirement for streaming alerts beyond the GCN notices scheme. As part of this we need to have defined the basic JSON/AVRO data schema to be used and place it in the emfollow/userguide. It is understood that work is in progress within the CBC/AllSky group in order to define the astrophysical quantities that will appear in GCNs and by extension to this data structure in discussion. Until such global discussion settles, copying the content of what we communicated during O3 and as part of the GCN notices is the point of departure. Upon further input from the broader discussion within the CBC/AllSky/LVK and a final decision reached, this data structure will need to be revised.