Document the complete mapping from EMBL API to DwC-A

[timrobertson100]

There is a start to this but on first glance it seems incomplete. E.g. the sequence description, the accession is listed as mapped to 3 fields which seems suspicious, and important IDs like study_accession, sample_accession and bio_material seem missing.

I suggest we document the complete specification including the extensions that will be used and also what scope the resulting datasets will be (e.g. is it a single, or multiple datasets for different types of deposits)