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/journal/src/journal.rs

@@ -13,7 +13,13 @@ use std::{
 };
 use workspace::{AppState, OpenVisible, Workspace};
 
-actions!(journal, [NewJournalEntry]);
+actions!(
+    journal,
+    [
+        /// Creates a new journal entry for today.
+        NewJournalEntry
+    ]
+);
 
 /// Settings specific to journaling
 #[derive(Clone, Debug, Serialize, Deserialize, JsonSchema)]