Add new visibility example.

This new example shows how we can have an invisible container with visible children.

docs/examples/styles/visibility_containers.css

@@ -0,0 +1,22 @@
+Horizontal {
+    padding: 1 2;     /* (1)! */
+    background: white;
+}
+
+#top {}               /* (2)! */
+
+#middle {             /* (3)! */
+    visibility: hidden;
+}
+
+#bot {                /* (4)! */
+    visibility: hidden;
+}
+
+#bot > Placeholder {  /* (5)! */
+    visibility: visible;
+}
+
+Placeholder {
+    width: 1fr;
+}

docs/examples/styles/visibility_containers.py

@@ -0,0 +1,30 @@
+from textual.app import App
+from textual.containers import Horizontal, Vertical
+from textual.widgets import Placeholder
+
+
+class VisibilityContainersApp(App):
+    def compose(self):
+        yield Vertical(
+            Horizontal(
+                Placeholder(),
+                Placeholder(),
+                Placeholder(),
+                id="top",
+            ),
+            Horizontal(
+                Placeholder(),
+                Placeholder(),
+                Placeholder(),
+                id="middle",
+            ),
+            Horizontal(
+                Placeholder(),
+                Placeholder(),
+                Placeholder(),
+                id="bot",
+            ),
+        )
+
+
+app = VisibilityContainersApp(css_path="visibility_containers.css")