Add example to grid styles docs, rename it

docs/examples/styles/grid.css

@@ -0,0 +1,20 @@
+Screen {
+    layout: grid;
+    grid-size: 3 4;
+    grid-rows: 1fr;
+    grid-columns: 1fr;
+    grid-gutter: 1;
+}
+
+Static {
+    color: auto;
+    background: lightblue;
+    height: 100%;
+    padding: 1 2;
+}
+
+#static1 {
+    tint: magenta 40%;
+    row-span: 3;
+    column-span: 2;
+}

docs/examples/styles/grid.py

@@ -0,0 +1,18 @@
+from textual.app import App
+from textual.widgets import Static
+
+
+class GridApp(App):
+    def compose(self):
+        yield Static("Grid cell 1\n\nrow-span: 3;\ncolumn-span: 2;", id="static1")
+        yield Static("Grid cell 2", id="static2")
+        yield Static("Grid cell 3", id="static3")
+        yield Static("Grid cell 4", id="static4")
+        yield Static("Grid cell 5", id="static5")
+        yield Static("Grid cell 6", id="static6")
+        yield Static("Grid cell 7", id="static7")
+
+
+app = GridApp(css_path="grid.css")
+if __name__ == "__main__":
+    app.run()