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/git_ui/src/git_panel.rs

@@ -77,11 +77,17 @@ use zed_llm_client::CompletionIntent;
 actions!(
     git_panel,
     [
+        /// Closes the git panel.
         Close,
+        /// Toggles focus on the git panel.
         ToggleFocus,
+        /// Opens the git panel menu.
         OpenMenu,
+        /// Focuses on the commit message editor.
         FocusEditor,
+        /// Focuses on the changes list.
         FocusChanges,
+        /// Toggles automatic co-author suggestions.
         ToggleFillCoAuthors,
     ]
 );