Add docs for layout property

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"
+```