Explain how to create a mapping of non-OpenTelemetry log data format to OpenTelemetry logs

[tigrannajaryan]

Produce guidelines for authors which implement a mapping from a data format to OpenTelemetry data model (e.g. Collector receiver authors), which tells where to put a particular piece of data they have.

We have examples of mappings here. We need to write down principles which were used to produce these and can be used to help produce future mappings for other formats.