Add docs for layout property

docs/examples/styles/layout.py

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+from textual import layout
+from textual.app import App
+from textual.widget import Widget
+from textual.widgets import Static
+
+
+class LayoutApp(App):
+    CSS = """
+    #vertical-layout {
+        layout: vertical;
+        background: $panel;
+        height: auto;
+    }
+    #horizontal-layout {
+        layout: horizontal;
+        background: $panel-darken-1;
+        height: auto;
+    }
+    #center-layout {
+        layout: center;
+        background: $panel-darken-2;
+        height: 7;
+    }
+    Screen Static {
+        margin: 1;
+        width: 12;
+        color: $text-primary;
+        background: $primary;
+    }
+    """
+
+    def compose(self):
+        yield layout.Container(
+            Static("Layout"),
+            Static("Is"),
+            Static("Vertical"),
+            id="vertical-layout",
+        )
+        yield layout.Container(
+            Static("Layout"),
+            Static("Is"),
+            Static("Horizontal"),
+            id="horizontal-layout",
+        )
+        yield layout.Container(
+            Static("Center"),
+            id="center-layout",
+        )
+
+
+app = LayoutApp()
+app.run()

docs/styles/layout.md

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+# Layout
+
+The `layout` property allows you to define how a widget arranges its children.
+
+## Syntax
+
+```
+layout: [vertical|horizontal|center];
+```
+
+### Values
+
+| Value                | Description                                                                   |
+|----------------------|-------------------------------------------------------------------------------|
+| `vertical` (default) | Child widgets will be arranged along the vertical axis, from top to bottom.   |
+| `horizontal`         | Child widgets will be arranged along the horizontal axis, from left to right. |
+| `center`             | A single child widget will be placed in the center.                           |
+
+## Example
+
+Note how the `layout` property affects the arrangement of widgets in the example below.
+
+=== "layout.py"
+
+    ```python
+    --8<-- "docs/examples/styles/layout.py"
+    ```
+
+=== "Output"
+
+    ```{.textual path="docs/examples/styles/layout.py"}
+    ```
+
+## CSS
+
+```sass
+layout: horizontal;
+```
+
+## Python
+
+```python
+widget.layout = "horizontal"
+```