Improve Installation/Configuration

I've tried to install it in the last few days, but it seems like there are missing variables in your example. Also you should explain what each variable is for, because some names are not very clear. Another thing I've noticed is, you set some environment variables but also include the .env file which contains those same values.

And one question, which I couldn't test: Can every user just add new mailboxes without any verification (except maybe via ALLOWED_EMAIL_DOMAINS)?

I archived this with my IdP directly (authentik, saw you on the discord with the same issue, do you want my solution?), so I probably won't try this again, but I wanted to share my experiences with it :)

Commencez par comparer les variables d’environnement décrites dans l’exemple d’installation avec les valeurs dupliquées dans le fichier .env ; identifiez les entrées manquantes et expliquez chaque variable. Inspectez ensuite le comportement d’autorisation lors de la création de boîtes aux lettres, notamment ALLOWED_EMAIL_DOMAINS, et documentez le résultat vérifié.