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 :)

Comienza comparando las variables de entorno descritas en el ejemplo de instalación con los valores duplicados en el archivo .env; identifica las entradas que faltan y explica cada variable. Después, inspecciona el comportamiento de autorización de la creación de buzones, incluido ALLOWED_EMAIL_DOMAINS, y documenta el resultado verificado.