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/workspace/src/theme_preview.rs

@@ -11,7 +11,13 @@ use ui::{
 
 use crate::{Item, Workspace};
 
-actions!(dev, [OpenThemePreview]);
+actions!(
+    dev,
+    [
+        /// Opens the theme preview window.
+        OpenThemePreview
+    ]
+);
 
 pub fn init(cx: &mut App) {
     cx.observe_new(|workspace: &mut Workspace, _, _| {