Added mention about language configuration for the back office

[mnocon]

Based on Support's feedback:

Hi, I have not found anything about admin_group.languages in the docs.

# config/packages/ibexa_admin_ui.yaml
admin_group:
   .. 
    # For admin this needs to contain all languages you want to translate content to on the given repository.
    languages: [eng-GB]

Probably this information should be added somewhere ? (...) Any (content-) language that is used in Admin Siteaccesses needs to be added imho.