gpui: Store action documentation (#33809)

Closes #ISSUE

Adds a new `documentation` method to actions, that is extracted from doc
comments when using the `actions!` or derive macros.

Additionally, this PR adds doc comments to as many action definitions in
Zed as possible.

Release Notes:

- N/A *or* Added/Fixed/Improved ...

crates/collab_ui/src/notification_panel.rs

@@ -74,7 +74,13 @@ pub struct NotificationPresenter {
     pub can_navigate: bool,
 }
 
-actions!(notification_panel, [ToggleFocus]);
+actions!(
+    notification_panel,
+    [
+        /// Toggles focus on the notification panel.
+        ToggleFocus
+    ]
+);
 
 pub fn init(cx: &mut App) {
     cx.observe_new(|workspace: &mut Workspace, _, _| {