feat: Added missing slides

slides/slide21.tscn

@@ -0,0 +1,23 @@
+[gd_scene load_steps=2 format=3 uid="uid://birgeyuu3aqk7"]
+
+[ext_resource type="Texture2D" uid="uid://gqcx5v7hfjj2" path="res://resources/backgrounds/slide21.png" id="1_8xrtu"]
+
+[node name="Slide21" type="Control"]
+layout_mode = 3
+anchor_left = 0.962
+anchor_top = 0.856
+anchor_right = 0.962
+anchor_bottom = 0.856
+offset_left = -923.52
+offset_top = -462.24
+offset_right = -923.52
+offset_bottom = -462.24
+grow_horizontal = 2
+grow_vertical = 2
+
+[node name="TextureRect" type="TextureRect" parent="."]
+layout_mode = 0
+offset_right = 40.0
+offset_bottom = 40.0
+scale = Vector2(0.5, 0.5)
+texture = ExtResource("1_8xrtu")