editor: Show hints for using AI features on empty lines (#20824)

Co-Authored-by: Thorsten <thorsten@zed.dev>
Co-Authored-by: Antonio <antonio@zed.dev>

Screenshot:

![screenshot-2024-11-18-17 11
08@2x](https://github.com/user-attachments/assets/610fd7db-7476-4b9b-9465-a3d55df12340)

TODO:
- [x] docs

Release Notes:

- Added inline hints that guide users on how to invoke the inline
assistant and open the assistant panel. (These hints can be disabled by
setting `{"assistant": {"show_hints": false}}`.)

---------

Co-authored-by: Thorsten <thorsten@zed.dev>
Co-authored-by: Antonio <antonio@zed.dev>
Co-authored-by: Thorsten Ball <mrnugget@gmail.com>

crates/zed/src/zed.rs

@@ -1,4 +1,5 @@
 mod app_menus;
+pub mod assistant_hints;
 pub mod inline_completion_registry;
 #[cfg(any(target_os = "linux", target_os = "freebsd"))]
 pub(crate) mod linux_prompts;