Add recommended prior knowledge of users to prerequisites // provide links for research

[shiz0]

Every now and then there seem to arise misunderstandings about what knowledge one should have to set up and operate a mailserver(suite) like mailcow.
I'd like to add an overview recommendation what knowledge the users targeted by the docs are assumed to have acquired beforehand.
Also, I think it would be nice to provide some links as starting point for users research on these topics.

I've created this issue to collect community input on

• general (mail server related) technical terms or fields that should be mentioned
• useful links to guides or other resources for self education on these topics

We cannot of course go into detail too much and mention every single command, but provide a rough overview.

[seniorm0ment]

Some ideas:

• Basic UNIX based system administration, security, and management
  • Basic navigation and tools of UNIX based operating systems ( ls, cd, vi, etc.)
  • Basic usage of UNIX based CLI tools, especially networking ones such as ping, dig/drill/nslookup, etc
  • Basic UNIX based server security such as SSH key management

the RHEL docs can be decent for some of these topics, for example security:
https://access.redhat.com/documentation/en-us/red_hat_enterprise_linux/6/html-single/security_guide/index

• Basic Docker/Docker-compose knowledge

• Basic networking understanding, and ideally experience with DNS records.

• Safeguarding, Backups, Restores

I'd be willing to help throw in some information when the actual documentation page is created.