Add example for grid columns.

docs/examples/styles/grid_columns.css

@@ -0,0 +1,11 @@
+Grid {
+    grid-size: 5 2;
+    grid-columns: 1fr 16 2fr;
+}
+
+Label {
+    border: round white;
+    content-align-horizontal: center;
+    width: 100%;
+    height: 100%;
+}

docs/examples/styles/grid_columns.py

@@ -0,0 +1,22 @@
+from textual.app import App
+from textual.containers import Grid
+from textual.widgets import Label
+
+
+class MyApp(App):
+    def compose(self):
+        yield Grid(
+            Label("1fr"),
+            Label("width = 16"),
+            Label("2fr"),
+            Label("1fr"),
+            Label("width = 16"),
+            Label("1fr"),
+            Label("width = 16"),
+            Label("2fr"),
+            Label("1fr"),
+            Label("width = 16"),
+        )
+
+
+app = MyApp(css_path="grid_columns.css")