Docs: Improve OS installation debugging sections

[SchoolGuy]

Is your feature request related to a problem?

As a user of Cobbler
I want to have it easier to identify issues during installations via Cobbler
so that I can spend less time debugging issues.

Provide a detailed description of the proposed feature

To enhance the user experience two things should be added to the documentation:

1. Add on the landing page a note that clarifies what content goes where. This should be done similar to what we have on the landing page of the GitHub Wiki.
2. Add a note - where appropriate - to our Documentation that says that it is helpful for debugging purposes to use serial consoles or BMCs to record the screen. Some information during the boot process is only visible for mere seconds and might even be overlooked by personnel that is experienced with installing OSes on different hardware. (An example would be messages from the firmware on the screen, those will be only visible for a second until the next boot entry is attempted.)

Alternatives you've considered

None

Additional information

This was requested in a Jitsi meeting that I had with community members that was organized via our Matrix/Gitter chat.

[SchoolGuy]

@tiltingpenguin could you try and take care of this in the coming weeks maybe?