Provide guidance on generating unique identifiers

[duncandewhurst]

The ITU's open fibre data for Brazil lacks unique identifiers for nodes. The node names are also non-unique.

We should consider providing guidance on how to generate unique identifiers when data sources lack them. The guidance could cover nodes, spans, phases and organisations. It might be as simple as suggesting that identifiers are assigned incrementally, e.g. "0", "1", "2", etc.