Add German translation (de) for Knuspr.de and Gurkerl.at #97

Add a German translation (de.json) for users of Knuspr.de and Gurkerl.at.

Contributions are very welcome! 🙌 No coding is needed: it's one JSON file. Comment here if you'd like to take it, so nobody else starts the same translation.

Steps

1. Copy custom_components/rohlikcz/translations/en.json to custom_components/rohlikcz/translations/de.json.
2. Translate the text values only. Keep the keys and placeholders such as {name} unchanged.
3. Where it makes sense, use the same terms as the Knuspr.de and Gurkerl.at app (cart, delivery slot, credit and so on).
4. Open a pull request.

Part of #96.