Add example app with content alignment grid.

docs/examples/styles/content_align_all.css

@@ -0,0 +1,39 @@
+#left-top {
+    /* content-align: left top; this is the default implied value. */
+}
+#center-top {
+    content-align: center top;
+}
+#right-top {
+    content-align: right top;
+}
+#left-middle {
+    content-align: left middle;
+}
+#center-middle {
+    content-align: center middle;
+}
+#right-middle {
+    content-align: right middle;
+}
+#left-bottom {
+    content-align: left bottom;
+}
+#center-bottom {
+    content-align: center bottom;
+}
+#right-bottom {
+    content-align: right bottom;
+}
+
+Screen {
+    layout: grid;
+    grid-size: 3 3;
+    grid-gutter: 1;
+}
+
+Label {
+    width: 100%;
+    height: 100%;
+    background: $primary;
+}

docs/examples/styles/content_align_all.py

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+from textual.app import App
+from textual.widgets import Label
+
+
+class AllContentAlignApp(App):
+    def compose(self):
+        yield Label("left top", id="left-top")
+        yield Label("center top", id="center-top")
+        yield Label("right top", id="right-top")
+        yield Label("left middle", id="left-middle")
+        yield Label("center middle", id="center-middle")
+        yield Label("right middle", id="right-middle")
+        yield Label("left bottom", id="left-bottom")
+        yield Label("center bottom", id="center-bottom")
+        yield Label("right bottom", id="right-bottom")
+
+
+app = AllContentAlignApp(css_path="content_align_all.css")