Update & fix C# blocks under getting_started/

- `_Process` take its delta argument as a double
- Node classes need to be partial
- Use `SignalName.*` instead of `nameof(*)` for signals
- Add a note about how `delta` arguments are doubles

getting_started/first_2d_game/03.coding_the_player.rst

@@ -487,7 +487,7 @@ this code to the function:
     public void OnPlayerBodyEntered(PhysicsBody2D body)
     {
         Hide(); // Player disappears after being hit.
-        EmitSignal(nameof(Hit));
+        EmitSignal(SignalName.Hit);
         // Must be deferred as we can't change physics properties on a physics callback.
         GetNode<CollisionShape2D>("CollisionShape2D").SetDeferred("disabled", true);
     }