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 :)

Beginne damit, die im Installationsbeispiel beschriebenen Umgebungsvariablen mit den in der .env-Datei duplizierten Werten zu vergleichen; identifiziere fehlende Einträge und erkläre jede Variable. Untersuche anschließend das Autorisierungsverhalten bei der Erstellung von Mailboxen, einschließlich ALLOWED_EMAIL_DOMAINS, und dokumentiere das verifizierte Ergebnis.