Add dock example with four docks.

docs/examples/styles/dock_all.css

@@ -0,0 +1,34 @@
+#left {
+    dock: left;
+    height: 100%;
+    width: auto;
+    align-vertical: middle;
+}
+#top {
+    dock: top;
+    height: auto;
+    width: 100%;
+    align-horizontal: center;
+}
+#right {
+    dock: right;
+    height: 100%;
+    width: auto;
+    align-vertical: middle;
+}
+#bottom {
+    dock: bottom;
+    height: auto;
+    width: 100%;
+    align-horizontal: center;
+}
+
+Screen {
+    align: center middle;
+}
+
+#big_container {
+    width: 75%;
+    height: 75%;
+    border: round white;
+}

docs/examples/styles/dock_all.py

@@ -0,0 +1,17 @@
+from textual.app import App
+from textual.containers import Container
+from textual.widgets import Label
+
+
+class DockAllApp(App):
+    def compose(self):
+        yield Container(
+            Container(Label("left"), id="left"),
+            Container(Label("top"), id="top"),
+            Container(Label("right"), id="right"),
+            Container(Label("bottom"), id="bottom"),
+            id="big_container",
+        )
+
+
+app = DockAllApp(css_path="dock_all.css")