docs(mailserver): update to mailu 2.0 (#17)

Steps to migrate from the previous version 1.9:

**Important**: You migrate at your own risk. Make sure to have a backup
of your mailserver in case something goes wrong!

- Back up your existing `.env` file
- Run `docker-compose exec admin flask mailu config-export --secrets
--output-file mail-config.yml`
- Run `docker-compose cp admin:/app/mail-config.yml .`
- Update `docker-compose.yml` and `.env`
- Carefully compare all values inside `.env` with `TODO: CHANGE ME`
annotation and set the value from your previous `.env` file
- If you made any custom changes to the `docker-compose.yml`, you need
to manually look at them
- Read the [mailu migration guide](https://mailu.io/2.0/releases.html)
and do any required changes
- Run `docker-compose up -d --remove-orphans`
- Run `docker-compose cp mail-config.yml admin:/app`
- Run `docker exec -i $(docker-compose ps -q admin) flask mailu
config-import -v < mail-config.yml`
- Run `docker-compose exec admin rm mail-config.yml && rm
mail-config.yml`
- Remove file `docker_postgres_init.sql`
- Remove folder `./mailu/postgres`

## Explanation

In the previous docs of our mailserver example, we used a custom
postgress database. But with version 2.0, the mailu team explicitly
discourages from using it and you should instead just use the default
sqlite. So most of the steps above are for migrating the mail-config,
see [mailu docs](https://mailu.io/2.0/database.html)

Author: larsrickert