Add icon to indicate nightly features in book

[zerosnacks]

Given the book targets latest nightly most users, who are now pinned on stable, can run into mismatching documentation

Adding an icon or emoji of some kind to indicate a feature is is part of nightly but not of stable will help highlight this. It will also help users see more easily what features are to be part of the next rc / stable release

ideas: 🌒 or 🌛

cc @grandizzy