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 :)

まず、インストール例に記載されている環境変数と、.env ファイルに重複している値を比較します。不足しているエントリを特定し、各変数を説明します。次に、ALLOWED_EMAIL_DOMAINS を含むメールボックス作成の認可動作を調査し、検証済みの結果を記録します。