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/main.rs

@@ -66,7 +66,7 @@ use zed::{
     OpenRequest,
 };
 
-use crate::zed::inline_completion_registry;
+use crate::zed::{assistant_hints, inline_completion_registry};
 
 #[cfg(feature = "mimalloc")]
 #[global_allocator]
@@ -401,6 +401,7 @@ fn main() {
             stdout_is_a_pty(),
             cx,
         );
+        assistant_hints::init(cx);
         repl::init(
             app_state.fs.clone(),
             app_state.client.telemetry().clone(),