Document the preferences.toml + history and favorites CSV files on the fossplant.re wiki before making any further extra changes

[marin-m]

[marin-m]

Done here for preferences.toml: https://fossplant.re/wiki/SongRec/Internal_formats/preferences.toml

Still to do for song_history.csv and favorites.csv: https://fossplant.re/wiki/Main_page#Project_documentation_(work-in-progress)