docs: add comments to UI components, HUD, and start screen (#65)

src/features/start_screen/mod.rs

@@ -4,6 +4,7 @@ use components::*;
 
 pub mod components;
 
+/// Plugin for the main menu screen.
 pub struct StartScreenPlugin;
 
 impl Plugin for StartScreenPlugin {
@@ -14,6 +15,7 @@ impl Plugin for StartScreenPlugin {
     }
 }
 
+/// Spawns the main menu UI.
 fn setup(mut commands: Commands) {
     commands.spawn((
         RootMarker::MainMenu,
@@ -60,6 +62,7 @@ fn setup(mut commands: Commands) {
     ));
 }
 
+/// Handles main menu button interactions.
 fn menu(
     mut commands: Commands,
     mut next_state: ResMut<NextState<AppState>>,
@@ -84,6 +87,7 @@ fn menu(
     }
 }
 
+/// Cleans up main menu resources.
 fn cleanup(mut commands: Commands, query: Query<Entity, With<RootMarker>>) {
     for entity in query.iter() {
         commands.entity(entity).despawn();