Document "preferences" in the Qbs context

Add a Preferences  topic that explains what preferences are in the Qbs context, and how they can be applied globally or to specific profiles, along with a listing of available keys and their functions.

This will enable us to refer to one place in the documentation from the other places where "preferences" are referred to. For example, "preferences" is the same as "profiles.<profile>.preferences" except that it applies to all projects regardless of the profile used. Global preferences, basically.