Documentation for aliases

# Describe the bug

Missing documentation of `/etc/maddy/aliases`.

# Steps to reproduce

I followed the tutorial to setup a simple personal mail server. It was easy and straightforward. Just the needed mail aliases took a long time to setup, because it was not documented and a search brought me at first to some complex setup descriptions. Eventually I found a mention of the aliases file in some PR or commit message.

# Log files
n/a

# Configuration file
n/a

# Environment information
* maddy version: 0.7.1

A sentence about `/etc/maddy/aliases` at the end of the tutorial and/or an example file for e.g. an alias hostmaster would be nice for newcomers.