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/title_bar/src/application_menu.rs

@@ -12,7 +12,15 @@ use smallvec::SmallVec;
 use ui::{ContextMenu, PopoverMenu, PopoverMenuHandle, Tooltip, prelude::*};
 
 #[cfg(not(target_os = "macos"))]
-actions!(app_menu, [ActivateMenuRight, ActivateMenuLeft]);
+actions!(
+    app_menu,
+    [
+        /// Navigates to the menu item on the right.
+        ActivateMenuRight,
+        /// Navigates to the menu item on the left.
+        ActivateMenuLeft
+    ]
+);
 
 #[cfg(not(target_os = "macos"))]
 #[derive(Clone, Deserialize, JsonSchema, PartialEq, Default, Action)]