Document how to pre-select a country when using Smart IP / GeoIP

[bojanz]

Sourced from drupal.org: https://www.drupal.org/project/address/issues/3043028

We could cover this in both Commerce and Address docs. The Commerce docs would demonstrate the code inside a country resolver, while Address could use hook_entity_create() to create an empty address with the known data (country + whatever else the service provides)