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 :)

首先，将安装示例中描述的环境变量与 .env 文件中重复的值进行比较；找出缺失的条目并解释每个变量。然后检查邮箱创建授权行为，包括 ALLOWED_EMAIL_DOMAINS，并记录已验证的结果。