I think you’re assuming that such a mapping is standardized. It isn’t.
The contents of an IDoc can vary by installation.
The contents of EDIFACT documents will vary by partner.
You, or an analyst, are going to need to work with the SAP folks and each EDIFACT recipient to specify the correct mapping. EDI documents are usually defined using Implementation Guides which specify the segments and data elements to use and the allowable values. One guide defines one message type and version (e.g. ORDERS for D97A).
Your company may define a guide for all vendors to follow or each vendor may require that you use theirs. Using the guides in conjunction with knowledge of the IDocs that will be used will allow a mapping spec to be created.