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/menu/src/menu.rs

@@ -12,13 +12,21 @@ pub fn init() {}
 actions!(
     menu,
     [
+        /// Cancels the current menu operation.
         Cancel,
+        /// Confirms the selected menu item.
         Confirm,
+        /// Performs secondary confirmation action.
         SecondaryConfirm,
+        /// Selects the previous item in the menu.
         SelectPrevious,
+        /// Selects the next item in the menu.
         SelectNext,
+        /// Selects the first item in the menu.
         SelectFirst,
+        /// Selects the last item in the menu.
         SelectLast,
+        /// Restarts the menu from the beginning.
         Restart,
         EndSlot,
     ]