Preference system should likely be documented in the code loading documentation

[KristofferC]

The way code is loaded via Project and Manifest is documented in https://docs.julialang.org/en/v1/manual/code-loading/. This makes it so one could write tools that create these files without e.g. reading the Pkg source code to find out how these are used.

It would be good to have a section about the preference system since that is also a part of code loading so that other tools could potentially be written for them (outside of Preferences.jl) and so that it is possible to understand how project files with preferences fields should look.