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/project_diff.rs

@@ -41,7 +41,15 @@ use workspace::{
     searchable::SearchableItemHandle,
 };
 
-actions!(git, [Diff, Add]);
+actions!(
+    git,
+    [
+        /// Shows the diff between the working directory and the index.
+        Diff,
+        /// Adds files to the git staging area.
+        Add
+    ]
+);
 
 pub struct ProjectDiff {
     project: Entity<Project>,