Add user-facing documentation

[sethaxen]

Currently the documentation only explains how developers should use the package, but users also need to know how to use it to update preferences. It took me some troubleshooting to figure it out. I think it needs to document the following 2 ways:

1. set_preferences!: this requires the user launch a new Julia session if developers implemented preferences with a const like in the ReadMe.
2. add an entry to LocalPreferences.toml in the user's environment

It should note that neither of these work with temporary environments, since relaunching loses the environment.

[vchuravy]

I think the first step would be to actually setup documenter :)

[claforte]

In the meantime can we at least get a link to an example project that uses Preferences.jl?

[claforte]

Also the README should mention that Pkg handling preferences correctly only came into being in v1.8.0, as mentioned in Preferences/test/PTest/src/PTest do_test()...

[nsajko]

It seems there's another package that aims to provide an interface more suited to interactive use: PreferenceTools.jl.