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/panel/src/panel.rs

@@ -5,7 +5,15 @@ use settings::Settings;
 use theme::ThemeSettings;
 use ui::{Tab, prelude::*};
 
-actions!(panel, [NextPanelTab, PreviousPanelTab]);
+actions!(
+    panel,
+    [
+        /// Navigates to the next tab in the panel.
+        NextPanelTab,
+        /// Navigates to the previous tab in the panel.
+        PreviousPanelTab
+    ]
+);
 
 pub trait PanelHeader: workspace::Panel {
     fn header_height(&self, cx: &mut App) -> Pixels {