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>

assets/settings/default.json

@@ -490,6 +490,9 @@
     "version": "2",
     // Whether the assistant is enabled.
     "enabled": true,
+    // Whether to show inline hints showing the keybindings to use the inline assistant and the
+    // assistant panel.
+    "show_hints": true,
     // Whether to show the assistant panel button in the status bar.
     "button": true,
     // Where to dock the assistant panel. Can be 'left', 'right' or 'bottom'.