Add docs for Placeholder.

docs/examples/widgets/placeholder.py

@@ -0,0 +1,39 @@
+from textual.app import App, ComposeResult
+from textual.containers import Container, Horizontal, Vertical
+from textual.widgets import Placeholder
+
+
+class PlaceholderApp(App):
+
+    CSS_PATH = "placeholder.css"
+
+    def compose(self) -> ComposeResult:
+        yield Vertical(
+            Container(
+                Placeholder(label="This is a custom label for p1.", id="p1"),
+                Placeholder(label="Placeholder p2 here!", id="p2"),
+                Placeholder(id="p3"),
+                Placeholder(id="p4"),
+                Placeholder(id="p5"),
+                Placeholder(),
+                Horizontal(
+                    Placeholder("size", id="col1"),
+                    Placeholder("text", id="col2"),
+                    Placeholder("size", id="col3"),
+                    id="c1",
+                ),
+                id="bot"
+            ),
+            Container(
+                Placeholder("text", id="left"),
+                Placeholder("size", id="topright"),
+                Placeholder("text", id="botright"),
+                id="top",
+            ),
+            id="content",
+        )
+
+
+if __name__ == "__main__":
+    app = PlaceholderApp()
+    app.run()