auto grid docs and test

docs/examples/guide/layout/grid_layout_auto.css

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+Screen {
+    layout: grid;
+    grid-size: 3;
+    grid-columns: auto 1fr 1fr;
+    grid-rows: 25% 75%;
+}
+
+.box {
+    height: 100%;
+    border: solid green;
+}

docs/examples/guide/layout/grid_layout_auto.py

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+from textual.app import App, ComposeResult
+from textual.widgets import Static
+
+
+class GridLayoutExample(App):
+    CSS_PATH = "grid_layout_auto.css"
+
+    def compose(self) -> ComposeResult:
+        yield Static("First column", classes="box")
+        yield Static("Two", classes="box")
+        yield Static("Three", classes="box")
+        yield Static("Four", classes="box")
+        yield Static("Five", classes="box")
+        yield Static("Six", classes="box")
+
+
+if __name__ == "__main__":
+    app = GridLayoutExample()
+    app.run()