add short section on using screen to the tutorial

[aghitza]

The question "how can I let Sage run when I'm logged out?" comes up often enough that it probably deserves a short section in the tutorial, most likely in Chapter 3 (The Interactive Shell).

CC: @sagetrac-mvngu

Component: documentation

Keywords: tutorial, screen

Issue created by migration from https://trac.sagemath.org/ticket/4204

[sagetrac-mvngu]

comment:1

CC'ing myself.