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/supermaven/src/supermaven.rs

@@ -25,7 +25,13 @@ use std::{path::PathBuf, process::Stdio, sync::Arc};
 use ui::prelude::*;
 use util::ResultExt;
 
-actions!(supermaven, [SignOut]);
+actions!(
+    supermaven,
+    [
+        /// Signs out of Supermaven.
+        SignOut
+    ]
+);
 
 pub fn init(client: Arc<Client>, cx: &mut App) {
     let supermaven = cx.new(|_| Supermaven::Starting);