Add docs for Placeholder.

docs/examples/widgets/placeholder.css

@@ -0,0 +1,50 @@
+Placeholder {
+    height: 100%;
+}
+
+#top {
+    height: 50%;
+    width: 100%;
+    layout: grid;
+    grid-size: 2 2;
+}
+
+#left {
+    row-span: 2;
+}
+
+#bot {
+    height: 50%;
+    width: 100%;
+    layout: grid;
+    grid-size: 8 8;
+}
+
+#c1 {
+    row-span: 4;
+    column-span: 8;
+}
+
+#col1, #col2, #col3 {
+    width: 1fr;
+}
+
+#p1 {
+    row-span: 4;
+    column-span: 4;
+}
+
+#p2 {
+    row-span: 2;
+    column-span: 4;
+}
+
+#p3 {
+    row-span: 2;
+    column-span: 2;
+}
+
+#p4 {
+    row-span: 1;
+    column-span: 2;
+}

docs/examples/widgets/placeholder.py

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+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()