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/language_tools/src/key_context_view.rs

@@ -13,7 +13,13 @@ use ui::{
 };
 use workspace::{Item, SplitDirection, Workspace};
 
-actions!(dev, [OpenKeyContextView]);
+actions!(
+    dev,
+    [
+        /// Opens the key context view for debugging keybindings.
+        OpenKeyContextView
+    ]
+);
 
 pub fn init(cx: &mut App) {
     cx.observe_new(|workspace: &mut Workspace, _, _| {