Add databags/alternatives.ini

This file is needed to support the new script to automatically enable preview languages in Lektor projects:

https://gitlab.torproject.org/tpo/web/tpo/-/blob/l10n-ci-dev/enable_incomplete_languages.py

It can also be useful to help ensure that each language's configuration is consistent between the different projects.

It's not meant to be symlinked directly like other files in this folder, so I'm not 100% certain this is the best place for it, so your ideas are welcome.