Specify which part of the ISO standard is supported for country codes

Feedback on [GBIF Data Processing](https://techdocs.gbif.org/en/data-processing/) ([Github source for this page](../blob/main/./en/data-processing/modules/ROOT/pages/index.adoc))

Should it be specified that GBIF supports ISO 3166-1 Alpha-2 and Alpha-3 country codes, and not principal divisions and subdivisions like ISO 3166-2?